The most common meter used in poetry is iambic pentameter. Why do so many poets use this meter?

dstuva is absolutely correct in answering that iambic
meter is widely used in English because it so closely matches the natural rhythm of that
language. Just consider a sentence or two, spoken in a regular tone, such
as:

I was walking down the
street....

You'll notice that, with a small exception here
and there, an English speaker will fall into an alternation of stressed and unstressed
syllables (such as "WALKing DOWN the STREET"). Of course, there are different kinds of
stresses (not all stresses are equal); even so, it's safe to see most spoken English
rhythm as iambic.

At the same time, however, I don't agree
with dstuva that pentameter somehow allows for more natural sounding verse than, say,
tetrameter. The traditional ballad stanza uses iambic tetrameter and trimeter and -- far
more than most sonnets, at least -- tends to very closely follow the patterns of spoken
English.