It is probably just rusted onto the hub. Were you able to get the other one off?
Couple things to try:
1) Whack the hub area with a hammer. Be careful not to hit a wheel stud or the area that the pad rides on (unless you arent planning to use these again, or plan to cut them before reuse....). This will help break the bond formed between hub and rotor.
2) Apply liquid wrench (or similar substitute) to hub area where the rotor meets. Reuse hammer from step 1.
3) If rotor still will not come off, a propane torch around the hub area will also help to make things easier. Again, use hammer on the hub area.
I have never had a rotor that would not come off after all of that. There are no set screws that hold these rotors on, so its a matter of force at this point.