It's hard to pick just one. If you have to, the biggest reason is perceived lower price vs. XBOne. PS4 is eating into XBox market share like Pac-Man. But there are other factors:
1. Wii owners bought a PS360. Early gen 7 adopters bought a Wii. Then they upgraded to PS360. After establishing Network ID's and friends, Achievements/Trophies, and just simply viewing PS360 as an upgrade over Nintnedo, they're slow to accept the idea of going back to the Wii brand.
2. PS4 launched at the same time as XBOne, not a year behind.
3. No bold new product from competitors. No Wii Sports, Gears, Kinnect, etc.
4. COD is now established on PS4.
5. PS4 is viewed as more powerful.
6. Nintendo has dropped the ball on software up until MK8.
7. Europeans love the Playstation brand.
8. Owners of PS2 who skipped PS3 are feeling nostalgia. (this was a boon to Wii in gen 7)
9. MS pissed off some core people with policies.
All of these things are a factor. Sony has yet to release killer software, and PS4 doesn't really do anything remarkable. But PS4 has an image as a superior piece of kit to XBOne at a lower price and Wii U has yet to grab a lot of attention, so that's all PS4 needs right now.
