Generators and transformers are always designed in kVA because designers never know about the load it will provide power to. Then how do we know the wind turbine generator will deliver 1500 kW or 2 MW etc?
In most practical contexts, the load of a wind turbine is pretty well specified: It will be connected to a regional power distribution grid. For the relevant purpose here the grid can be considered an ideal AC voltage source -- pushing 2 MW into it will not affect its voltage amplitude or waveform much.
It is therefore reasonable to design the generator to work in that specific electrical environment -- and, in particular, to avoid delivering significant reactive power to the grid (which would tend to make distribution companies unhappy). So specifying its capacity or operating limits in watts is appropriate.
Generators are designed for a specified capacity that includes both a kVA and a kW capacity. Wind turbines are designed for a specified capacity to drive a turbine that would include the torque that can be provided at a given speed over a specified range of wind speeds and rotating speeds. What is known is the power that can be delivered at any wind speed. A generator of a given capacity will not be installed unless there is a reasonable expectation that there is sufficient demand for power to make the installation economically advantageous.