I think Mongoose got it about right (bar the Pentax bit of course
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People generally buy into a system when they start off for some reason that's sound at the time, I got a Nikon camera because I'd used a Nikon D2H in my last job and they had a couple of lenses I could borrow. Similarly I bet a lot of people buy into Canon or Nikon because of people they know already having gear they can borrow and learn from, a friend of mine did that, he bought a Nikon D3000 for £200 and has borrowed my 18-55mm kit lens and my 70-300mm. Its enough to get him going until he can buy his own lenses. The other major factor is probably cost, I haven't looked into it but I seem to remember people saying that Canon were slightly cheaper especially for lenses.
There's no denying that each system has its positives and negatives, Nikon have the better ISO range at the mo, Canon have higher MP and a larger lens range (especially a good range of F4 lenses) although Nikon are starting to wise up to it and hopefully we'll see a good range from them too.
But really there's not that much in it, I bet you can't look at a picture and say it was taken with a canon or a nikon (or a pentax for that matter) without looking at the EXIF.