Syria has been an ally of Russia since the end of the Second World War. It is natural that Russia would want to support its ally. Also Tartus in Syria is Russia’s only naval base in the Mediterranean and only Assad guarantees its existence.
You might ask why the west didn’t do more to support the opponents of Assad especially after saying Assad was unfit to lead and had to go. This seems to be because the rebellion became increasingly Islamist and a threat to the wider region and so we bombed ISIS and indirectly helped keep Assad in power.