It's not. Some people don't have the time or inclination to purchase 2 consoles that base between them costs over $900 just for the base. And for the couple asshats that are like "I can afford both" it's not anything to do with affording, its just a choice of time vs value. While there may be a few who have no worries (because obviously you can spend as much time as one can playing games) not everyone out there can. One console is always going to be sitting. No different than deciding to get a game on PC or console. (but at least with the PC it can be doing other useful things).
Therefore, which console is the predominant console IS of note. The thing that people forget though is even 1 game can change that if it is the right game. There also used to be a style difference. There used to be an actual reason to purchase one console over another or even a PC. Different consoles had a different game direction/style attached to them. And think about PC games, they were a completely different style as well.
I think starting with the 360/PS3 that kind of went away and with this latest batch of PC wannabes, it's gone completely they all have the exact same games short of a few exclusives, that look more or less like every other game out there then they are ported to PC. There is so little diversity that it pretty much comes down to "which looks/performs best" most of the time. Or...if your friends are lazy slob gamers too, then who all has them so you can play with them.
(and no offense to all the lazy slob gamers out there )