Because of simplicity of the generator and because the output of an asynchronous generator is always syncronised with the grid.
I think is called an asynchronous generator because the output frequency of the generator is not syncronised with the rotational speed of the generator put the frequency of the grid
And if you want to learn more about WTG I would suggest this site
today only a low percentage of the newly installed power is set up with asynchronous generators. Most of the large size units use either synchronous generator or a double fed indcution generator. The machines are connected to the mains by converters. Maybe people in US didn't realize that trend because such units couldn't be exported to the US due of the fear of patent infringment lawsuits.
The main problem with induction generators is their reactive power requirements and their inability to stay connected to the grid during voltage dips. There is new technology now from GE and soon from other manufacturers that addresses the voltage ride-through problem. Visit:
