If you don't understand I'll find Sklansky's explanation. Its much better.
Here's why you fold.
Lets say you are 70% to win vs 2 players. If you win 70% of the time you will still only have around 1k chips the other stack(s) will be much higher than yours and you still have virtually no shot at second. The 30% time you lose you are gone.
If you fold.
You have a flat 50% chance at second place money.
If you call you have a .3(50)% chance at second place money(depending who wins side pot)

If you call and win remember you still have a VERY small chance to win first. Its almost non-existant.

FOLDING will make you 2nd place much more often, which is really all you can expect ehre.

__________________
“There's no sense in being precise when you don't even know what you're talking about.” - John von Neumann