NCAA Football Players!?

So in the CFL (Canadian Football League) Draft, they were able to draft players from NCAA Football such as Scott Mitchell from Rice, Carl Volny from Central Michigan, Marco Iannuzzi from Harvard. So my question is how come they can draft some football players from the NCAA, but not the big ones like Cam Newton (Auburn), Von Miller (Texas A&M), Marcell Dareus (Alabama), etc...?