Why did topher grace leave that 70s show?

Answer 1

He wanted to concentrate on his movie roles that he was offered at the time. He was offered the plum role of Eddie Brock/Venom in "Spider-Man 3." Being cast in "That '70's Show" would make him an instant overnight TV star. But dollars to diamonds to anything else, he would be forever associated with that one role--TV's most loveable square bland beige dork-in-a-half, Eric Forman. But this one role, he would make the transition to bone-fide movie star. He'd be known as "Topher" and not just "Eric." He'd have a career after "The '70's" ended.

Answer 2

Topher left because he wanted to do movies, full-time. He did movies while he was in That 70's Show, and got some great reviews. It has also been said that he did not like being compared to his character, Eric, all the time. He wanted to be known as Topher Grace, an actor.