It's not that we forget history. It's that those who believe in the socialist fairy tale don't teach it and pretend it doesn't exist, so that if you want to know it, you have to hunt it down for yourself.
Even while the socialists were killing millions in Russia, our news media refused ever to print the truth. Had they done so then, I think the world would be quite different now. As it is, most of the media still refuse to tell the truth, and it is looking more and more like we really need to suffer the ravages of socialism here before people see the light. Even then, many still will blame capitalism, they have been conditioned so well.
People want to believe it can happen, regardless of the ample evidence of the hulks of burnt out socialist tanks that line the Yellow Brick Road to the future of Socialism.
Communists do not often believe their own dogma, recognizing that the dogma is for the proles to swallow, and their own dogma is privilege and power. And people (like many gullible and stupid people in Western Europe and America) watched the failures from afar and said "They didn't do it the right way. If they do it right, it will succeed."
The disturbing part is that many of the people pushing socialism believe in their hearts they can "do it right this time". In Communism, the dry-eyed ones at the top are dependent on the useful idiots with fevered foreheads, blazing eyes, and spittle on their lips to move the needle for them. In the wonderful world of socialism, the ones at the top are fever-minded, eyes-blazing and spittle lipped. They believe their own crap right up to the point they find themselves on a gibbet or meat hook.
Socialism: The Lady In Waiting for Communism to rape her.