But there aren't. It's an illusion of diversity.
They're all owned or being bought up by the same centralized handful of outfits-turned-conglomerates that have always controlled the means of distribution. Haven't you noticed the flurry of mergers (successful or not) in the last 10-15 years?
Indeed. A good example is how much trouble Snowden had in finding newspapers willing to publish him. Same goes for Wikileaks.
If you want to talk about a "marketplace of ideas," this is just an example of how it works in practice. Alex Jones or users of /r/FatPeopleHate etc. are 100% legally allowed to spread their ideas in most countries and therefore still participate in this "marketplace". However, consumers themselves find certain actions and ideas repulsive and last year's young and vulnerable males have perhaps grown up a little, so websites are trying to read the market and get ahead of the curve before a smart competitor can pick up those consumers by advertising "we don't allow content like that on this platform, but you can find it on Patreon if you really want."
If the concerns about Patreon's censorship that we're seeing in this thread were really as valid as their prevalence should indicate then Reddit banning subs like /r/FatPeopleHate and /r/CoonTown might have led to Reddit's downfall a long time ago (like how very many redditors predicted), and Voat.co would be a flowering bastion of ideas and content from people drawn to its promise of unadulterated free speech. Instead Voat is a second-rate, hostile finge community while Reddit is only gaining prominence as a website. That doesn't seem to be changing anytime soon.
The same thing would happen to any "alternative" Patreon, so for very many powerful members of this "anti-feminist"/"new right"/"Intellectual Dark Web" I think they might see this as writing on the wall for them and their entire movement. Maybe what they're afraid of in the long run is that their ideas simply don't stand up to scrutiny and without access to a sufficiently large mainstream community where the law of averages would budget them a steady stream of converts they would steadily lose members and just fizzle out on some forgotten corner of the internet.
There has always been a realm of discourse that society agrees upon and that we use to set the boundaries of conversation, just a few decades ago anti-Semitism and racism were allowed and now it isn't, overall this is a good thing.
There are more platforms than ever available to you if you want to express your fringe ideas, each of the platforms has the right to ban you if it goes against what they stand for. Everyone who uses the platforms understands the risks, I don't understand where all this pearl clutching is coming from.