I'll tell you what I think the major factors are. One, because basic PSN is free, yes. Two, because Microsoft so badly bungled the launch, and inertia is so very important. Inertia is the only reason we're still stuck with Windows; they got up a head of steam and managed to use it to build themselves a relatively unassailable position. Their stranglehold is only now failing on the desktop, and only because they went full spyware.
When Microsoft first launched their console in this generation, they said all kinds of idiotic things and had to backpedal on them even while the launch was occurring, and that cost them dearly. They have simply never recovered due to network effects. If your friends have the PS4, you're more likely to buy the PS4 so that you can play the same exclusives, share controllers, etc etc.