I think there's a few things coming together.
For one thing, in common with various other online fandoms, English-language alternate history communities are overwhelming full of young, straight, white, cis, Anglo-Americans. Many people get interested in high school; the most popular timelines are overwhelmingly military or monarchist or both, the sort of history that a socially awkward teenage boy often develops an interest in.
There's also an inherent tendency to wish fulfilment that comes out in the community; as Orwell wrote in Notes on Nationalism, "Every nationalist is haunted by the belief that the past can be altered. He spends part of his time in a fantasy world in which things happen as they should – in which, for example, the Spanish Armada was a success or the Russian Revolution was crushed in 1918 – and he will transfer fragments of this world to the history books whenever possible."
Many of the posters in the online community exhibit this tendency- you see it in the Paradox gaming community, the Other Place, any number of subreddits or deviantart sites. Rhodesia could have, should have survived with US backing! The Kaiserreich deserved to win the Great War, and it would have been a happier world if they had. State's rights. The Desert Fox could have taken Cairo, and he wasn't a real Nazi, and if he was he was a good one who would have stopped the bad people once he'd become Fuhrer.
It's not entirely a disease of the right. There are certain PODs which bring out a utopian Leftist thinking which often ends up waving away authoritarian tendencies in the belief that the People's end justifies the means of ending large groups of the People; the Kaissereich setting used to be quite bad for that and the Spartakist mod for HOI4 embraces the old idea that the evil SPD stopped the world from a happy and peaceful and socialist and democratic Germany that was there for the taking.
But the vast majority of the time, wish fulfillment timelines tend to be the product of people who feel themselves to be cheated out of a rightful place in the ruling class. In many ways, it's the same pressures that produce the alt-right. The leftists say that white men have it easy, right? But you're sitting at a computer in your parent's house, your job is crappy, you're unhappy, you're sexually unfulfilled, and if white men really did have it easy how come it's so hard for you? You're smart, smarter than other people. Just unappreciated.
There's another world though which is more fair. And it's not about hating anyone else, really. Look at the mess Mugabe made of Zimbabwe- you don't think those people should never have had equal rights- they could have had them eventually, once they were ready for them. What was needed was leadership, a place where people like you could do the things that needed to be done. Hard men making hard choices, right? And being rewarded. Because once you'd come in from your tour with the Rhodesian Rangers or whatever- and in this fair world you were tough enough for that, I mean you've always been tough enough but you just need training- people would know that you're a real man, a real leader. And actually, by the way, blacks had always had equal rights- some of your best friends were black. They supported Ian Smith, really. And they supported Marse Lee, too.
It's not fair that it's not true, because it is true, it's more true. So keep picturing it, keep imagining it, keep wondering what it would have taken for that perfect world to exist. It should exist, and you should live there.
In a way, you already do.