If I'm not mistaken, signal is specifically hosted on AWS to so that governments can't just blacklist their IPs without blacklisting all of AWS. So moving to bare metal isn't really an option.
They can, but the collateral damage of doing that is high, which is a deterrent. The only regimes where that's realistically an option are ones where they've consciously developed an independent tech ecosystem (e.g. China) or where their power is so secure they couldn't care less about the damage.
Russia might fall under the category of "regimes...where their power is so secure they couldn't care less about the damage," and it appears they did in fact see a lot of collateral damage during the attempt. India's blocks look to be localized an temporary.