Just FYI - Allied Force was in 1999 and /r/AskHistorians abides by a "20 Year Rule", which attempts limit topics of discussion that still may be ongoing or be considered "current events". Also, you should know that "what if"-type questions are frowned upon here as well.

That said, you're asking why Russia, which was economically dependent on Western states, banks and international institutions following the ruble devaluation of 1998, which had recently been defeated in a war inside its own territory decided not to enter a war against NATO? Doesn't the answer write itself?