The haiku mother language is Japanese. Now, its publication language is English.
Haiku was introduced into the Anglo-Saxon culture in the beginning of this century ( between 1910 and 1917). It caught attention from poets like Ezra Pound, James Joyce, D.H. Lawrence...
After that there were collections (translations of H.G. Henderson 1934, 1958, R.H. Blyth 1949).
The appeal of haiku was also due to the Americans' interest in zen philosophy, that grew after the contact with the Japanese culture after World War II.
During the Beat Period, Allen Ginsberg, Gary Snyder and Jack Kerouac wrote haiku, what contributed to diffuse it in the American culture. After 1963, haiku magazines were published :
American Haiku, then Brussels Sprout, Frogpond, Inkstone (canadian), Modern Haiku.
Why that greater diffusion in North America ?
The following hypothesis can be proposed : the Americans' attraction by the Japanese culture after World War II, the influence of the Beat Generation, but also the Transcendentalists' influence at the end of last century ((Henry David Thoreau, Walt Whitman, Ralph Waldo Emerson). They developped a peculiar philosophy, close to Zen Buddhism.
For more information : see the text ""Haiku in english in North America" from George SWEDE at http://www.atreide.net/rendezvous/histnotham.htm
Haiku is taught as poetry in the United States primary schools. No surprise that it has a place in the cultural landscape
English as a lingua franca
Haiku was born in the linked poetry Japanese tradition, even if it is an only piece. It is discussed, imitated, completed. Internet is an extraordinary medium to share and exchange it.
English is lingua franca on internet. It is normal that haiku grows and is exchanged in this language. For us, French speaking people, English must have the same role as Latin in Middle Ages as lingua franca, to effectively diffuse our way of seeing the world. It is not to erase our linguistic membership but to use another language aside our culture. That is, for me, a plus, not a renunciation.