Because last time I used it, it decided all of a sudden not to download absolutely anything ever again, which I feel to be a critical part in a torrent program.
Also, at that time, it lacked some pretty critical features like "no, I don't want to download everything", "perhaps I'd like to download this into a different drive/location than ~/Downloads" and few others.
Probably has changed but why change when Transmission now does everything I've ever wanted the way I want it.