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.