To see why, have a look at The Platygaen Qu'ran.
It was not always so.
At the peak of their culture the Muslims did not believe in a flat earth; owing to their extensive translation of Greek materials, they accepted the Ptolemaic spherical-earth view.But the rise of Islamic Fundamentalism has caused such innovative, modernistic notions to be discarded.
In 1993, Sheikh Al-Azeez bin Baaz, the supreme religious authority of Saudi Arabia, declared that the earth is flat, and that anyone saying otherwise is an atheist deserving of punishment.Few Judeo-Christian theologians - even fundamentalist ones - still credit the view that the Earth is disc-shaped, and separated from waters above and below by a hemisphere of beaten metal (see illustration), rather like those small paperweights you can sometimes see, the ones with snowflakes in them.
 Yousef M. Ibrahim, "Muslim Edicts take on New Force", The New York Times, February 12, 1995, p. A-14. (NYT Online archive only goes back to 1996), quoted in The New Statesman and elsewhere.
I was very careful to check out the bona fides of this fatwa. There's already too much misinformation on the net. I've yet to find a copy in English of the good Sheik's Magnum Opus "Evidence that the Earth is Standing Still" published by the Islamic University of Medina in 1974, (the ones with the lines
If the earth is rotating as they claim, the countries, the mountains, the trees, the rivers, and the oceans will have no bottom and the people will see the eastern countries move to the west and the western countries move to the east.on page 23 ) but the fatwa seems pretty much proven.
More discussion on the Islamic Flat Earth (or not) is at the Al Bawaba, Islamic, and TakingITGlobal Fora (Forums).
For what it's worth, the few Muslims that I know personally are more like this guy:
This style of "keep-your-mouth-shut", suppress other opinions, islam could never produce such a Golden Age of diversity and over-all tolerance as too many good-hearted people are too quickly finding out.