The main reason Donna hung on, I think, beyond Anna Stuart's many gifts, is because she never really "won." She was always being punished and suffering for anything she did wrong. The nadir was probably when she had sex with Jake after Michael harshly dumped her (for her own good, of course...) and they had the story of Stacey blackmailing her into giving up the son she had adopted, then her affair was exposed on the witness stand at Marley's trial for shooting Jake. We were, presumably, meant to sympathize with Michael and Stacey, but they were both such unbearable and sanctimonious assholes that it was just about impossible. So this meant that Donna could be haughty, or bitchy, but was never untouchable, and viewers could still care.
Even to the end of the show, Donna continued suffering and paying, losing every man she loved, and being abused by an unhinged Marley. She was one of the few characters who got no real happy ending.
Hutton was unconventionally handsome, charismatic, and also had some good comedy skills we got to see every once in a while. I never remember if it was his choice to leave or not, but he was a find.
I thought Marcus Smythe (RIP) was also good in the role, but the writing was awful. Just as he became more interesting he left the show, never to return...and only rarely to be mentioned.