Why should I as OSS maintainer give a cut to Tidelift and even let my revenue be solely dependent on their algorithm when I could just put a onliner in my project stating that usage for other OSS is free and commercial users need to buy a license with a link to an online payment page where I control price, duration, etc.?
Because selling stuff requires more effort than just "here's my price, pay me".
By doing that, you've put your code into the realm of "commercial software", which engages with companies/corporate use in a whole different modality. If you've never sold software to a business, you will have no idea how much is involved in this side of things.
In addition to being treated differently by your potential customers, you will also earn the hatred and ire of your open-source-loving colleagues. Some bored college kid will see your dual-licensed software as an immoral act, and spend time building a less-awesome and more incomplete "completely free" version, which will then attract dev mindshare and users and eventually ossify into a de facto standard which you will then have to support.
That college kid will one day graduate, and think he can build a business on top of this amazing software he's made that everyone loves and wants to use, and then sit around trying to figure out how to make money from it. He may even consider making a "free-mium" model or an "enterprise" offering on top of the "open core" of his widely adopted OSS.
These will most likely fail, because - again - SELLING THINGS IS HARD.
Then you and this college kid who disrupted your dual-licensed OSS will one day meet at a symposium for "open source sustainability". It will be awkward. Teeth will be gnashed. You will get lectured about not having just used Patreon. Meanwhile the companies that use your and the college kid's OSS continue to hit their quarterly numbers for Wall Street. Executives earn out bonuses. The circle of life continues.
This "charge for commercial use" model was tried -- shareware -- and was an economic failure. It's advocated against by most organizations, like FSF and Creative Commons.
At most you see a "this is GPL but I wrote it so I'll give you a license to not follow the GPL for my code". Which is a quite different thing.
Setting up a payment page and writing one line to the license, even if I outsource that one time effort, will be indefinitely cheaper than having my revenue at the mercy of some random algorithm which doesn't even take into account how important my software might be to a company and giving a huge cut to a third party which really doesn't do anything other than profiting of my hard work. I struggle to see the appeal for OSS maintainers.
For one thing, Open Source software can't require commercial users to pay. The common way is AGPL then, in the assumption that it's unacceptable for companies to comply with, and selling a different license too. It's also a hurdle to take that someone makes a decision to pay for your thing specifically (and doesn't go search for a free replacement). Lots of still quite valuable components don't really meet that hurdle, and you severely limit your adoption by using e.g. AGPL. If (and that's a large if) tidelift gets somewhat widely adopted, it'd trickle down to all parts of the ecosystem, not just the few standout pieces being able to get paid otherwise.
