According to some statements by the Russian authorities, it appears to be
A. The destruction of Famillies due to the cruel shunning policy
B. What we would term 'hate speach' toward the state and other religions
C. The blood ban putting Russian lives at risk
Of course, there are further reasons they don't talk about publicly.
Such as the fact they are a specifically American cult (like Scientology & Mormonism, also banned) which fleeces Russian citizens of their hard earned cash and sends it back to a(rich) American cult.