For me it was because I got a bunch of hand-me-down lenses to get me started.
To be honest, I kinda regret going with Canon, but at this point replacing all my lenses with Nikon equivalents would be costly.
Canon cameras lack a lot of capabilities that I would really like to have.
I'd like to know too.
Besides the basic trait of Nikon differentiating performance tiers by resolution, and Canon by AF; there are no features I can think of that Nikon has, that Canon doesn't have on their equivalent model.
One stands out to me: (more so because there are Third Party hacks for it) Nikon Mid range bodies can do more than 3 AEB's, something that I have always wanted. Canon does 3 AEB for all bodies except the 1D series, where it allows 7 AEB. Why not allow a 5AEB in the mid-range line (Yes ML gives us that option, but it should be standard).
That was the only feature I could think of too, but since ML lets me do 9, I disregarded it.