The reason is that there hasn't been enough collective pain outside of Ukraine. The pain is theoretical. Specifically, with the US, there hasn't been a large scale compelling reason sold to the people of the US as to why we should intervene substantially in this war. That will not happen in the next three years, given the current admin. That's just a fact. And it may be that our people as a whole are too ignorant or plain stupid to get it. But this isn't a problem unique to our times.
Recall that "America First" was a pre-WWII thing. People were adamant that Hitler was Europe's problem. We didn't enter the war in Europe until we were attacked - by the Japanese. Entering the war in Europe was just a tag on to that because Japan was aligned with Germany and Italy. This was a recognition by FDR's admin that getting involved in a European war was a politically dicey proposition for him. The current admin and much of the American electorate stand in the same posture today. The lessons of 1933-45 have been mostly forgotten, except for followers of history. Very few who were alive then are left to tell the tale. While that is sad, it is understandable. What is crazy to me, is the position the US has gained as a world power since 1945 - that is the world order that was formed as a result of WWII that has allowed us to become the richest, most powerful nation on earth - is being tossed away because a bunch of know-nothings have decided that this whole post 1945 order that we built to benefit ourselves, but also benefits others - benefits those others too much for contemporary tastes. It's self inflicted empire destruction.
tl;dr - It isn't happening because history repeats itself, and that includes Americans being insular and unconcerned about what takes place abroad.