1. A massive nuclear power
2. On the border of China
3. Willing to take risks for geopolitical gain that the Europeans would never dream of
4. A natural enemy of China since China claims (unofficially) large areas of Siberia including Vladivostok (their only major port on the Pacific).
If Russia were an ally, that would put our alliance to the north, east, and south of China. More importantly it would put the threat of a land based attack on the table which is currently not the case.