1) Europe was already cutting off oil imports from Russia. Even if at the time Europe was Russia's biggest customer, it wouldn't be that way for long. If a pipeline costs a lot to maintain and it will stop being useful soon, it makes sense to shut it down
2) Putting pressure on Europe to stop supplying the war -> (potential) less supplying to Ukraine -> Ukraine has less power -> advantage to Russia. Russia also gains an advantage in diplomacy. At least, this is probably what was expected. It backfired quite a bit.
3) Again, it wouldn't be the largest for long. And even if Europe would continue to be a major supplier, Nordstream wasn't the only pipeline supplying Russian gas to Europe. There's a multitude of them. There's even a few that go to Germany that haven't been blown up yet.
4) America has no major incentive to destroy Russia's gas infrastructure. Europe was already turning away from Russian gas, and so therefore America had a growing market there, which would continue to grow at an exponential rate. Exploding cheap gas pipelines doesn't quite make people want to go for more expensive gas sources like, I don't know, sending it across an entire ocean, and instead get oil from areas like the Middle East.
On the other hand, both actual combatants do actually have a reason to blow it up. Russia's is more diplomatic and Ukraine's is more economic, but they're both reasons nonetheless.