It wasn't so much that Tennant didn't want to do other things.. it's that he didn't want to leave like Tom Baker did. For those who don't know, Baker was basically fired. The series was going nowhere, and the BBC wanted to refresh it. Baker , at the time, had quite an attitude with regards to the show.. so they basically killed him off in favor of Tom Davison.
The reason Tennant wanted to leave now was , in his words "Because I could see myself doing this forever. I don't want to hate this years down the road." He , in short, didn't want to become another Tom Baker.