Why does almost everyone neglect Barbara Holland?

Why does almost everyone neglect Barbara Holland?

Did the rest even cared in start?

Barb seems to be the least popular girl in school and shown to have only single friend Nancy. And Nancy did all that for her only.

Mr. & Mrs. Holland had very minmal role, so we don't even see them much. And she presumably believed Nacny's lie that's why she was not worried that time But Mr and Mrs Holland do go to Wheelers' house after they know about her being missing:

During the sensory deprivation tank scene Eleven confirmed that Barb is dead and we can see how that affected Nancy and now she just want to kill the monster in revenge.

Nobody told anything about Upside Down to her parents so we can't know how they are dealing with it, most probably in grief of her absence behind the scenes.

They just pass her, not even attempting to resuscitate her

Was there anything left to resuscitate?

Barb was no different then other corpse to them and grieving for her seems irrelevant as both don't even know her well. Will is her son, she will for sure do way more for him then some random corpse. And Will was in better state then Barb at many level and don't show sign of decomposition like her:

We don't see any implication that her parents are grieving when Will is found or after the one month timeskip.

Because it doesn't matter for the creator to show them grieving, maybe they will in second season or maybe not.

Spoilers from second season

We got introduced with Murray Bauman, disgraced journalist-turned-conspiracy theorist who was personally investigating the Hawkins mystery and then later got hired by Barb's parents. Which also cleared that Barb's parents still thinks she is missing to that point.

Even Nancy was so pumped up to give Barb justice and she did got succeed with the help of Jonathan and Murray to get revenge from DoE. And Barb get a official funeral.

But it seems that Barb was supposed to be a small character only but her popularity and justice for Barb made them mention her again more prominently.

In Season 2 Barb's parents have put their house up for sale to fund a PI to investigate her disappearance. Nancy and Jonathan get proof of what happened to Barb and with the PI release it to the public and it shuts down the lab. It isn't the full truth that's made public as that would be tough to believe. Instead, they give the explanation a chemical from the lab killed Barb and the company covered up her death. Steve and Nancy have been having weekly dinners with Barb's parents as they are still clearly obsessed with finding her and haven't accepted that she is gone. The guilt that they're selling their home to find her deeply affects Nancy and she snaps at Steve for not caring and carrying on lying when they know what happened.

Although this isn't yet a strong answer, the following may be a point of interest, depending on what it exactly means.

We know there was huge campaign for: Justice for Barb and the Duffer Brothers did promise this would addressed.

As one of the previous answer's sites, season 2 does feature a subplot in which Nancy seeks to avenge Barb and does address the state of Barb's parents. However season two also does some other interesting things by giving viewers a decant and morally conscious scientist in Dr. Owens, and despite Brenner's missteps and abuses towards Eleven (and Eight), it once again reinforces that the scientist are only partially responsible for Barb's death, as it was Eleven whom accidentally blew open a hole to the other dimension and that the scientist themselves do not understand the nature of the Upside Down. This makes the actions of Nancy not necessarily better for Hawkins and it's residents, because we see Dr. Owens sincerely or genuinely trying to be helpful and be more honest about what they do and do not know. So in theory, should Dr. Brenner still be alive, the Hawkins Lab, now under investigation, may remove Owens and reinstate Brenner, seeing Owens as a bigger screw-up. (The reason this relevant is because it remains to be seen how Nancy's choices towards wanting 'Justice for Barb' effects everything, which may tell us, eventually, if this action really is justice or not--subjectively, speaking.)

The other interesting things the season may have done to touch on Barb might have to do with "B" alliteration. Two of the new characters, Billy and Bob both have "B" first names, with Bob being in parallel by being a somewhat geeky innocent character that didn't deserve to die from spawn of Demigorgon, and whose name is similar sounding to Barb's. Curiously Bob also tells Will about his dreams, being intimated by an antagonistic-figure, which he refers to as Mr. Boldo (again "B" alliteration).

In addition Nancy comes across a person that looks like Barb from the back, whose not Barb (possibly a 'Ghostbuster' allusion or pun -- and Nancy is also wearing "Barb" colors, as is the Not-Barb) and when Eleven goes and stays with Eight in the warehouse, there the name "Barbelith" graffitied on a wall, which is also a reference to Grant Morison's The Invisables comic. (I will come back with a screen cap, if I can find one).

Note: Scientist have to go into Upside Down to fix Upside Down version of a satiate to get there equipment to better work.

In The Invisibles, Barbelith is the name of the "placenta" for humanity; a satellite-like object located on the dark side of the moon. It recurs throughout the story as a supernatural moon seeming both intelligent and benign. Barbelith's role is like that of a placenta in that it connects the hologram of our subjective reality to the realm outside of our space-time, the domain of the magic mirror, and helps humans to realize their true nature beyond the subjective concept of "self".

Grant Morrison describes its origins as follows: "The word 'BARBELiTH' is derived from a dream I had when I was about 20 or 21 and coincided with my first structured 'magical' experiences and a minor nervous breakdown (in the dream, BARBELiTH was the name of some higher dimension or alternate reality)." https://en.wikipedia.org/wiki/Barbelith

So it's unclear what all of these things add up to. Whether it might be the Duffer Brothers trolling fans, paying homage to Barb by continuing to acknowledge her both in plot and in subtext, or if the Barbelith is pointing to some kind revenge story for a "mutated" monster-Barb (thinking it could also play on a Hell allegory to Lilith) or of Barb's consciousness is preserved in the Upside Down somehow????

Some additional info on the original intent of the character Bob, and the changes made, have come to light through interviews with The Duffer Brothers. I'm adding this on, because their reason for the change also includes the purpose of Bob, as it relates to Joyce, by siting that his death is a motivator for her to now avenge him--solidifying my suggestion that Joyce is then in parallel to Nancy, and thus Bob is in parallel to Barb.

“That was never on the table. We had talked about the death of some major characters, that may or may not happen in the future near or far. But that was never part of the discussion for Season 2. The death of Bob was initially much earlier. In fact, in an early outline, Evil Will kill him in like Episode 3.”

“We wanted to keep him alive longer, and then use his death as Joyce’s engine,” Levy said. “The avenging of that would become Joyce’s engine for the finale.”

http://collider.com/stranger-things-2-bob-death-original-version-will/#images