There are many reasons why this is happening IMO:
- Russia is using a small fraction of its firepower, according to expert reports I've seen.
- The reason, contrary to what we may all think in the West, may be that they are actually trying to avoid looking too evil with the whole world watching. Could they throw a tsar bomb in Kiev and be done with it? Probably, but the rest of the world wouldn't like that at all.
- They were unsure how the West would respond, and may believe that military retaliation was a possibility, so they decided to keep their big guns at home and "probe" Ukraine more slowly.
- They may be intentionally "hiding" their capabilities in order to keep an element of surprise when it really needs it (in Ukraine or if the fight escalates, with Western powers).
- Alternatively, of course, they may be just incompetent, which for some reason is what the West is thinking. Maybe that's what they want you to think?