Biggest reason with all the early adopters is always the same
Satisfaction with the previous console / brand
Wii --> WiiU : Low satisfaction, less people migrating forward with any urgency
PS2 --> PS3 : High satisfaction, but Sony bungled the messaging, priced too high, 3rd party better on X360 (cheaper & better)
PS3 --> PS4 : High satisfaction, more people migrating forward with confidence
X360 --> XBO : High satisfaction, but MS really bungled the messaging, priced too high, force-included kinect, 3rd party better on PS4 (cheaper & better)
Once a machine has momentum people know that is the "most" successful of the bunch, and then they know that will be best supported by software makers, and thats all that really matters. So unless XBO exclusives give huge bumps to XBO, don't expect the PS4 lead to erode/slow compare to the others.