Two reasons: 1) for about ten years, the US has viewed Russia as a direct adversary, for a variety of reasons, and because of that we 2) seek to weaken and limit Russian power by diplomacy and by proxy. This means the US is looking for ways to weaken and reduce Russia's ability to cause trouble. The easiest and least costly way to do this is to pull away Russian satellite states that are economically important to Russia. This was attempted in 2020 in Belarus, 2021 in Kazakhstan, Armenia, etc but the big prize was Ukraine- which had much of the Russian defense plant, was deeply economically intertwined with the Russian economy, and hosted its Black Sea fleet. That's the diplomatic/intelligence moves. The US also will supply forces close to Russia's border with military equipment. The whole purpose of supplying Ukranian forces (without our intelligence, money and arms, they would have lost) is to weaken and overextend the Russian state. The end goal is regime change probably, but simply reducing Russia's ability to project power is more realistic. This is just grand chessboard power politics, plain and simple- Ukraine's form of government does not matter.
