You haven't really followed Timmins career have you? He was a stud prospect and was seen as a key piece of the Avs future D. When he did play he looked good but unfortunately he couldn't stay healthy, and eventually was moved in the package for Kuemper (which many Avs fans were upset about). He got injured some more and Arizona is also a clown show organization so he wanted to leave, and Dubas fleeced Armstrong for him. He's finally healthy now and has looked good whenever he's in the lineup.
Timmins is very talented and has lots of potential, he just needs to actually play, but if he comes close to anything he was touted to be we have a potential top pairing D on our hands. The Leafs also have few up and coming players for the future and they already traded Sandin so it would be foolish to get rid of Timmins too and be left with bare cupboards.