Hmm... You could also ask why so many conservatives supported right-wing dictators - like Pinochet in Chile, Franco in Spain and Marcos in the Phlipines... and the less said about the conservatives - and the rich - support of Hitler from 1933 to 1939 the better - for the conservatives. We all have things we're not proud of. But I digress...
I guess most of the reason was that the liberals - or if you like the socialists - saw that capitalism wasn't working very well, and often made things worse for the poor... that the majority would never succeed under the capitalist model. So they welcomed anything different, and hailed it as socialism - like Israel and it's kibutzes... (until they (socialists) realized what happened to the Palestinians.)
Remember the goal of communism - which was the goal - was never a strong, paranoid Government, lead by a strong man... using secret police and an iron fist to prevent counter-revolution. The goal was to dismantle all government and let the people in the communities govern themselves by together deciding what was best for their community - real communism. Unfortunately, it always stopped at socialism in the form of a "we know best" government dictatorship.
I guess such movement will always attract the power-hungry... and that absolute power corrupts absolutely. And an other problem was that sometimes the ideology-based theories didn't work in practice in the real world (of course we see the same in capitalism - ie. Reagan's "new economy").
Anyway, the liberals/socialists in other countries, partly ignored the tales of atrocities coming out... partly hoped way too long for the leaders to give power back to the people... and partly had become too invested by what they'd already said and done, to just abandoning their support - even when it was obvious things had gotten bad. Add to that, that they probably still thought capitalism to be worse.
Edit/Add-on: Also in Europe "socialist" and "labor-unions" are not dirty words, most countries have strong socialist/labor-parties - many of which at least had some connections with the Soviet Union and the Communist Party... although often just breifly. And the Nazi-occupation in many countries, certainly "cured" many of too much leaning toward the right. Finally, due to the devastation in many countries during the war, there was a "lets pull together" attitude after the war, which may have strengthen socialist ideas (if not the communist ones).