TL;DR because they got couped and are having a civil war right next to your border, are ignoring political attempts to resolve their issues, and want to join a military alliance that is an extreme threat to your national security

Ukraine has been in civil war since 2014 and hasn't respected either of the ceasefires that it signed onto (Minsk protocols). The fight was broadly started over whether Ukraine should align itself with Europe & America or Russia. Since then, the country has been couped by the US and has been persecuting and killing Ukrainians who support Russia and/or protest the new government.

There's also how Ukraine wants to join NATO, which would certainly mean that the US would station nukes right next to Moscow. Remember the Cuban missile crisis? This is basically the same thing but with even closer territories.

I'm not certain that Russia entering the war was the best move, but I'm also confident that US interference would never stop peacefully. I'm also certain that if it were a western country in a similar position, our media would be framing it as an intervention instead of an invasion. Keep in mind that Russia and Ukraine are separate countries in the first place because of US meddling and they've only been apart for ~30 years (many Russians have friends and family who died in the civil war prior to 2023).