I echo your sentiment, if Russia was truly interested in Serbia’s interests she would have done more much earlier. However, I must qualify my statement to say that the Russian boots on the ground do care.
In 1999 Russia was very weak and the countries in between blocked the potential transports.
Yet Russians managed to arrange compromise in which Russian and NATO troops under UN supervision were to occupy only Kosovo for a limited time. After stabilization Serbian administration was to be restored. The West did not keep the deal.