Why isn't Magneto more powerful?

Because X-Men is not The Last Airbender?

Because the amounts of these metals in the body is far to small, remember the security guard had been injected with a lot of iron (this is about one of the most ridiculous scenes I have ever seen!).

Also because the metals that you list are not magnetic (they only generate a tiny magnetic field)

With respect to the comments below, where there was a discussion on what Magneto's powers were in the movies, I would use the most recent film Days of Future Past as more evidence for Magneto only being able to control metal. Whilst the continuity of this film is slightly dubious, it seems to still all be in the same timeline. Now there is a scene

where Magneto adds metal to the sentinels in order to control them.

I think it's clear that if he could control things that were not metal then there would have been no need for this to be done.

Probably for the same reason that humans are not really magnetic to start with: The amount of various metals in a human is actually quite small. I checked out this Wikipedia entry on the composition of the human body, took the metals you mentioned and checked to see how much all those metals together weigh. The total amount of all those metals in the average human body weighs roughly 24 grams. Put another way, it's .05 pounds of total metal. That's 5/100ths of a pound of metal. If you take a look at the metal balls (I saw at least 3 that Magneto had pulled out of the guard), each one of those had to weigh significantly more than that. According to this website, ONE lead ball would weigh nearly 40 times as much as what Magneto could pull out of a human. And although in theory, Magneto could simply yank the metal out of every human in the area to get enough metal, his prison was designed specifically to prevent him from being able to manipulate any metals outside the large room he was in. So every guard in the prison would have to walk up to Magneto for him to be able to pull all the trace metals out of everyone to do the same thing.

Not to diminish the work put into the other answers here, but the plain and simple answer is that it's a useful (albeit less-canonical) plot device. Perhaps they made him weaker to make him feel more human, thus more relatable. Or maybe they have been intentionally vague about the limits of his power to allow for more plot flexibility.

Some might argue that the films take place in a separate universe than the comics, which is a valid argument. But either way, in terms of continuity, the films are a mess. So if we want to know the true power of Magneto, we have to turn to the comics for help:

At the macroscopic level, Magneto's maximum effective range is unknown. However, in Avengers vs X-Men, he demonstrates the ability to manipulate magnetic fields at interplanetary distances. So I'd really hesitate to say you could build a prison large enough to hold him.

He is also capable of manipulating electromagnetic energy at the microscopic level. E.g. he can generate lightning bolts. All objects that have a greater than absolute zero temperature emit electromagnetic energy in the form of thermal radiation, which obviously includes humans. So the 'not enough iron in the blood' argument doesn't hold much water. As an example, from Vision and the Scarlet Witch vol. 4, he possesses the ability to manipulate photons to such an extent that he can actually bend light around himself, becoming effectively invisible.

He has also demonstrated incredible control over gravity. In Excalibur vol. 3, he bends space and time to create a wormhole through space.

In the movie versions, we ask, "What can Magneto do?" While in the comic versions we ask, "What can't Magneto do?!"

Magneto in comics is a near god, it's actually very hard to believe the X-Men winning fights with him, I think the film producers deliberately weakened him to give the heroes some chance against him. Even so, Magneto in the films still displays incredible power, like lifting submarines and entire bridges, which he has done in the books.