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.
Iambs are thought to be the formal rhythm that most closely resembles natural spoken and written English. A different form of rhythm may occur more naturally in another language, but in English it's iambs.
The enotes Study Guide seconds this thought:
Iambic rhythms come relatively naturally in English. Iambic pentameter is the most common meter in English poetry; it is used in many of the major English poetic forms, including blank verse, the heroic couplet, and some of the traditional rhymed stanza forms. William Shakespeare used iambic pentameter in his plays and sonnets.
Of course, being endorsed by Shakespeare, not to mention Chaucer, probably contributes to the prevalent use of iambic pentameter as well.
The length of a pentameter line enables a more natural-sounding line as well, if the poet chooses to make it so. The use of enjambment, for instance, can create more naturally sounding sentences with much less awkwardness, than is possible in, say, a tetrameter line pattern. Rhyme is also more easily separated in a line of ten syllables than it is in a line of six.
while writing,a poet mostly try to express his thoughts in a way which suits him best..i mean in which he can express himself easily..then one has to think about the reader..if a poet will not be able to make his meaning clear then it will be of no good..he wants to make others feel the same thing as he feels or to alter their viewpoint.a poet has to think about public taste also (what the public wants from him)..Iambic pentameter in my viewpoint is less complex and easy to understand..anyone, whether a poet or reader, can easily handle this..it also has the power to say more in less words..personally i really like it.. :-)