What
nobody has mentioned is that the
5-7-5 format
conveys considerably
more information in the original
Japanese than it does in
English. I believe someone or other has come up with a
syllable structure for
English Haikus that more closely represents the form of a
Japanese Haiku.
The extremely short length of English Haikus seems to be part of their charm however, and the 5-7-5 English format has been around so long now that it is unlikely it will ever change. This is probably why most people haven't even heard about the 'more faithful' other format.
I guess Japanese is just better for Haiku composition. Japanese Haikus have the same short rhythmical quality that a Haiku in English has, but convey a richer data set.