The GameGear had a MASSIVE technical superiority over the GB and we all know how that turned out.
It's all about user base, though, and this time the tables are turned on Sony. Just like in the console business, where even though the XBox and Gamecube are MUCH more powerful than the PS2, the PS2 gets by far the most support because of it's user base. This time, though, Nintendo has the immense user base (the GBA is almost a monopoly) and it's Sony who has the tiny little user base. Besides, technical superiority isn't much considering I haven't heard any developers complain about the GBA's lack of power. Hell, even it has some 3D polygon-pushing engines out now.