Because the People of Interest to Russia happen to use Telegram, not Signal.
Why is it easier to assume Russia forced Apple to block Telegram worldwide, a privilege Apple haven't even given China, than that there is another reason for the delay?
Russia hasn't banned Signal, has it?
Russia has not banned Signal yet.
A possible reason may be that
a) Telegram is still way more popular
b) Telegram was (and still is) a major networking service for the Russian opposition