Superior Tracking is the #1 reason. Absolute 1:1 sub-milimeter latency free volumetric tracking. It's incredible
All in one package. No add-ons or delays. You get everything. Oculus Touch will be another 100-200 USD, to be sold at an unknown date.
Room scale. This is an add-on to the tracking. Oculus can't hack it at room scale. They recommend to their developers to target forward facing experiences, which is basically standing, stationary, with a 180 degrees of freedom. That's not the VR I want.
Oculus have also been super shady about their practices, especially with exclusives and I refuse to support them.
Valve on the other hand have been wide open. Anti-exclusive, pro-user. Their devs have complete free reign with their devkits, while Oculus devs are under NDAs like you wouldn't believe.