It is not linked to the past, it is a rejection trauma.
It is considered normal in our societies to be with someone. And as a man, you are culturally expected to be able to find a woman, and you are responsible for it. If you are a single man, it is because you choose it or because you're a failed man. The media like to talk about how you now have sex at 11 and how men have many relationships. When you're still virgin at 18, you already feel like a miserable failure. And the longer it goes, the harder it become. For some men they don't even need to be virgin, they may have had their first relationship by luck.
Then came #metoo. Many men discovered how hard women had it because of men. And this broke something: before, when you were bad with women, you merely had to man up. There was only one model for men, but it was there. After metoo, this model was broken. Now men are completely helpless.
This second point is reinforced politically by the fact that the left spend a lot of time to talk about women problems, but never talks about how men should be to be successful with women. You have feminist movements spreading hatred against all men, and this is also hard. When communication makes it like all men are assholes and can't behave with women, but no one tells you how you should do instead, you just feel attacked and helpless.
With that, these men are completely destroyed. Suicide is an option an this point. But hatred is another one. And fascism lives out of hatred. But fascism came later. First there was hatred as an exit of this situation for the desperate men: it's not their fault, it's women fault. That's only the only way you can go out of the misery. And with hatred, you can start to blame them for everything that's happening. You're single because the women are stupid and prefer the machist men. And feminism is a fraud.
And oh boy do fascists love this situation! Now they can easily radicalise you because you already did the job. Because the progressive left abandoned these men, the fascists gladly took the opportunity. And you now have a strategy and a market around this.
Not all men fall for this. But the pain and hardship is shared by many men nonetheless. The problem is societal: there is no counter-model to the one old toxic one that's been discarded by #metoo. You can't merely make your own model, because the point is to be attractive to women. Which means it actually is a problem for women too. But the progressists unfortunately forgot those men in their fights for the rights of the sexual minorities and the women, so these men are alone.
As a man, you no have 3 solutions : you are lucky enough to be with a woman already ; you embrace the toxic masculinity of the fascists ; or you're left alone in a no man's land of solitude and despair. The incels are the men going from the 3rd to the 2nd category.
Society is changing. The process will take time with highs and lows. We're getting into a low with this problem, and we'll go to a new high when we will have fixed it. But for now, incels will be the symptom of the failure to include men in feminism.