Because Ukraine is making noises about throwing them out. The Black Sea fleet has technically been stationed in a foreign country since the end of the Cold War. There's an agreement that grants them the right to use Sevastopol until 2017, but after that nothing is guaranteed. There's a lot of hostility between Yuschenko and Putin right now, starting with the fomer being poisoned during the elections and continuing through the Russian natural gas pricing issue. If things continue as such, there's a chance the fleet could be out on its ear in a decade. Ukraine has also expressed interest in joining NATO, and having the Russian military stationed within its borders could be a barrier to that.
So all things considered, it makes sense for Russia to do some window shopping for a new port facility. And as their policies in Lebanon and other regional hotspots come under fire, Syria could benefit from a strong military alliance with an aging superpower.