I guess it could be that it's easier to build. You have a tower that only gives structural support and you have a gondola that only deals with all the rotation. In general it is difficult to have something that rotates and also carries a large load. And the rotating gondola technology is a solved problem. Plus you have a large rotor area that is always facing the wind instead of half your blade area going opposite to the wind. These are just guesses though.
I guess it could be that it's easier to build. You have a tower that only gives structural support and you have a gondola that only deals with all the rotation. In general it is difficult to have something that rotates and also carries a large load. And the rotating gondola technology is a solved problem. Plus you have a large rotor area that is always facing the wind instead of half your blade area going opposite to the wind. These are just guesses though.