Two reasons, I think.
1. Baathists have always had a close relationship with Russia going back to the Cold War.
2. Russia has always taken upon itself the protection of the Christian community in the Middle East. In fact, that is what started the Crimean War. The Baath Party was started by a Christian and has always protected the Christian community.
Of course, there is a third reason which applies to almost everyone. Who is there that can replace Assad?
I suspect that after ISIS is destroyed there is really going to be only one solution for both Syria and Iraq and that is partition, much as I hate the thought. The alternative seems to be genocide.