Why Canon? For me it was the quality of professional service and support.
It was in sheer frustration with Nikon's service and support that I ditched my Nikon F4 film cameras and switched to Canon EOS 1n. Canon CPS was a revelation.
The switch to Canon was further validated in the early digital days with clearly better cameras. The D30 of 2000 and the D60 of 2002 were revolutionary. The original 1Ds announced in late 2002 and subsequent 1-Series incarnations have mostly been clear winners for me.
OMG the 1Ds was almost a decade ago! http://www.dpreview.com/reviews/canoneos1ds/
Switching platforms today would be business suicide with a deep financial committment to Canon glass. But if I was starting out today, my research could possibly push me towards Nikon. Like Canon they're making brilliant cameras. And we all benefit from the hot competition!
But hey, in spite of the lack of stellar wide zooms, (Nikon 14-24 anyone?) I love my Canons and the brilliant support from CPS.