Think of it like this-- Sony is a major movie studio, film distributor, they are a major, major record label, they are a consumer electronics company (tvs, dvd players, stereo equipment, walkmen, laptop/desktop computers, etc.,) they are a major game system manufacturer/seller... and, oh... by the way, they also make cameras, although it's a much more recent venture for them, and their higher end cameras came about by them buying Konica/Minolta. They do have resources, and can invent and make things. They even make sensors, chips and other kinds of things. But, what's their main focus, and corporate culture and identity? Are you, as a photographer, their main concern? Probably not.
Now contrast that with Nikon - much smaller company, primarily optic and photographic oriented. Cameras and pro systems are who they are, and have been for many decades. Canon, while a bigger corporation and making office equipment among other things, is also a long-time major photographic manufacturer, and have been with Nikon, the leaders in most SLR technology for pros.
Pentax is a genuine camera maker, but they're small and losing market share. It shakes out basically as Nikon and Canon if you are professional and are using dSLRs--probably same folks who would have been using their old 35mm pro gear, it's just an evolution of an existing corporate identity and mission. Sony doesn't have that.
