Reasons are probably many
Why Russia might want to break use of Microsoft products, I came up with these possibilities
1) concern that stuff (state secrets, commercial secrets, Putin's secrets) will go via Microsoft to US state and its allies
2) not wanting to pay for it
3) money leaving country is bad for Russian finances
4) dependency on potentially unfriendly country
5) home-grown O/S path could be sold (forced) onto Russia's allies
6) cocking a snoot at US/"The West"
7) one of Putin's political enemies handles Microsoft licencing in the CIS
8) make Russia - as supporter of Open Source - look like "the good guy" in Open vs Closed argument
but you'd need a Kremlinologist to rank the probabilities.