Because last time when we were ceding territories of other countries to appease a dictator, it was year 1938 and we were ceding parts of Czechoslovakia. Guess what? It did not worked and WW2 started a year later.
Really, is it so hard to learn from a recent history?
Who is 'we' here. The US didn't do anything in 1938. And the Brits and have done so lots of times since then. And lets not pretend the US doesn't have a long history of ceding issues to dictators if convenient. Handing over Afghanistan to the Taliban is apparently totally ok, but not defending Crimea is like the Münich conference.
Looking at isolated cases in history you can come up with lots of example for anything and everything. Its a typical politician move to pick the singular events and then act as if it is a universal lesson for every situation.
One can make the argument, but just say 'Münich' as if it settles everything is dumb.
So what will happen with same appeasement in Ukraine? After few years of "peace" war will continue again. So you lost with appeasement and you got war again.
Oh yeah we already did it, 2014 was supposed to be "enough", but 8 years later, here we are, dictator wants more. Rinse and repeat
Really, is it so hard to learn from a recent history?