One reason is they are usually cheaper and less complicated for these relatively small generators. Secondly, an induction generator is more tolerant of variations in the wind speed without going unstable since it operates asynchronously from the power system. A synchronous generator must always turn at synchronous speed, and therefore large, rapid variations in the wind can cause it to slip out of step, usually resulting in a trip.
click on Wind Generators under Products
that will bring you to the wind generator manufacturers and fairly detailed generator specifications. The above link is specialized to Alternative Energy products and info.
The induction generator is simpler and more reliable power source when aligned with the windmill prime mover.
