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.