Why don't they cover themselves in zombie guts more often?

I'm going to convert my comment into a better answer. Unless it's explained in the comic series, there is no explanation. But, if we look at certain facts from the show we can speculate as to the reason to a point that could make sense.

• The first attempt failed (started to rain)
• That type of scene may have been too much for audiences. It was pretty grotesque.
• The group encountered a herd while on the freeway and hid under cars. They were left alone meaning that covering themselves with guts isn't necessary. More on this below.
• Once they reached the farm, there was no reason to use this technique as they only encountered one or two stragglers until the very end but at that point, they could only run.
• In season 3, they reached the prison and again, there wasn't any reason to use this technique. They wanted to clear the zombies from the yard, not sneak past them.
• In season 3, Michonne shows us that you can "disarm" a zombie and use them to protect yourself. It's long lasting and effective without requiring you get all gooey.

The show has a set of rules for Zombies but there are several articles that point out that the show doesn't follow the rules at all times. In this article Are The Walking Dead's Zombies Following the Show's Own Rules? the author specifically points out the guts scene with contrast to the freeway scene. Rule #6 says "Zombies have poor eyesight but they do have a strong sense of smell." If that's the case why were they able to get by just hiding under cars? The response was

"The smell thing that we saw in the first season is really just a rudimentary sense [the zombies have] that we don't smell like a dead person in some way, so they have some sense of smell that enables them to differentiate people from zombies. But they're not bloodhounds, you don't see them walking around and sniffing, following their nose or something. So hiding under a car is going to work."

Which seems to contradict rule #6. I bring this up because if we examine the way the story is going, the show is no longer about zombies, but human interaction and society in a post-apocalyptic world. So it's ok if the rules are broken because zombies are just there every now and then to remind us how scary the world is.

Another question would be, why not wear zombie guts when going on runs? It would make runs far safer. If we go back to rule #6 and it's contradiction, it would suggest that only a small amount of guts should be sufficient to deter the walkers. But, this isn't a 2hr movie, it's an ongoing series so making every scene safe would be boring.

In the end, I say 3 points to consider an answer are

• There wasn't another reason to do this
• Zombies don't matter as much as they did in the first few episodes of season 1
• Fear factor for audience

Of course, this is only speculation on my part.

Edit: Thinking about the contradiction to rule #6 again, even if it did start to rain on their first attempt, if they were able to hide under cars, why would a little rain cause them to become noticeable. A little guts should have been enough for them to get by. Food for thought.

Edit 2: Having reread the response to rule #6 contradiction, proximity could be a factor here. Covering yourself in guts may only be necessary when in very close proximity as they were in that scene, but hiding under cars and using disarmed zombies at a distance will keep them away. But, if proximity is the answer, why did Andrea get followed when walking back to Woodbury? She was further away from them than the group when they were under the cars AND with the noise of the truck, they should have been distracted away from her. We've seen members of the gorup walk past a single walker and not have any trouble.

That question was also directed to Robert Kirkman (The Walking Dead comic book creator and also a writer for the TV show), in Talking Dead S05E08 (aired after S06E08 "Start to Finish", the mid-season finale*). His response was (paraphrased slightly):

Well, look. One, it's not very practical. Being in close contact with that material is also gonna cause all kinds of illnesses, aside from any kind of zombie-related nonsense. So you don't want to be doing that all the time.

Two, it's not going to last. You would have to be replenishing those zombie guts constantly because they're gonna dry out. They're gonna lose their nastiness. They're not gonna smell. It's just not a practical way to go about things.

* Where Rick's group attempts to escape a walker horde by covering themselves with walker guts.

If you've EVER smelt a rotting corpse you'd understand. You NEVER acclimate to that smell...EVER. We actually have a VERY hard time acclimating to that scent, and a few others (vomit comes to mind)...if we ever can.

Plus ANY open wound would INSTANTLY become infected and you would have to SERIOUSLY scrub after to prevent this. ANY mucus membrane (urethra, anus, eyes, mouth, nose...etc) would become infected regardless of scrubbing. You think MRSA is bad NOW...