PS4 is selling because it's the best 8th gen console, with most power and speed, with a good price, and already the most top rated games (fo shizzle):
PS4: 5 games 90+, 35 games 80+ Metacritic
Wii U: 4 games 90+, 36 games 80+ Metacritic
XB1: 1 games 90+, 15 games 80+ Metacritic
Additionally because PS3 was awesome with GOTY hits and with PS+ giving access to a lot of AAA games on instant game collection.
People know Wii was a one night stand, compounded by Wii U delays in much needed 1st party titles, and subsequently losing most 3rd party.
People know X360 had RROD and bad DVD drives, but they loved it as a gaming machine. However most recent memory is X360 didn't get the games support that PS3 had in it's later years. That, compounded by f'ed up XB1 reveal, high price, and no compelling Kinect 2.0 software.
