Why can't the shuttle be a part of the ISS?

Answer 1

The Shuttle IS a part of the ISS whenever it docks. It becomes another large area where the astronauts can move back and forth. But they can't dock it there forever, otherwise, what are they going to come home in? They can't just roll down a window and jump out and fall back to earth...

I think what you might possibly be asking is why they don't leave a space shuttle permanently attached to increase the living area of the ISS. And the answer is, the space shuttle is NASA's Space Transportation System. After Challenger and Columbia were destroyed, NASA has only 3 left, and they are far too expensive to leave them attached to the ISS, they are needed for more NASA missions. Bringing up a docking module that is designed to stay there is a much cheaper way to increase the size of the ISS, and that is precisely what they do.