As you can tell from the slight bitterness in this thread, he was a pretty streaky scorer with the Leafs, and when he wasn't doing that, he didn't contribute very much in other areas of the game. As it often goes with those types of players, he appeared not to be giving a 100% effort at times, which particularly didn't sit well with fans after the Paul Maurice/Ron Wilson eras of watching guys just coast, collect a paycheque and seemingly not give a ****.
Mainly it was, I think, just an issue of team culture, and Carlyle preferring someone like Clarkson or Kulemin to play that role of the 'glue' guy next to Kadri/Lupul. By the end, Mac was often playing on the third/fourth lines, where he obviously wasn't going to score all that much and, as a result, bordered on useless.
He always came off like a good guy and a team player though, so I'm glad he's found his niche in Ottawa. He just wasn't a fit here, for various reasons.