Originally they were going to make Spider-Man 4, but Raimi dropped out of the project and with him both Tobey Maguire and Kirsten Dunst went. My guess is that Sony had allotted a budget for the fourth movie and simply used that for the reboot instead when Raimi dropped. The announcement of the reboot and the cancellation happened virtually simultaneously so I'm quite sure this was the case.
I wouldn't mind a change in casting and/or director. Heck, Peter Parker is the easiest character to cast in the world, he has no distinguishable features other than... brown-ish hair. And anyone reading the comics would be used to the characters being portrayed extremely different depending on current writers and artist. So a change in cast wasn't really a big deal. The part that pisses me off so much is that they do the origin story again. They just HAVE to have it occur during the highschool period. Sigh.
(<font size="2">Edit by Ashley: no need for gratuitous gory cartoons! :) Thanks!</font>)