While there are many factors, an essential one is that someone in Twitter finds you morally acceptable.
If Twitter thinks you're not morally good, they will actually remove a blue check which they previously granted (e.g. they did this to Milo Yiannopolous). If blue checks were just about identity veritifaction, this would make no sense since Milo's account's identity was never in question. Ergo, it isn't just about identity verification.
This has been my experience as well. A perfect example is podcasters - Anna [1]
hosts a relatively popular and occasionally politically inconvenient podcast called Red Scare. No check. Alexandra [2], the host of Call Me Daddy, also a popular comedy podcast, naturally has a check. This is pretty openly discussed by e-celebs of middling fame. Until one reaches a critical mass of popularity, you won't get a check if you don't correctly toe the culture line.
Did Milo lose his verified status because someone didn't like him or was it because he broke some rule in the T&Cs? IIRC he was a big part of "gamergate", a movement that acted hostile to a subset of Twitter's user base.
If it was about identity verification, the process should certainly not be reversible for "moral" reasons. Rather, I would expect some notability cutoff as the requirement for the blue letter. As it stands now, the manner in which it is distributed better reflects if Twitter (the org) likes you. That it generally also verifies your identity is secondary.
Sure, but this is barely different than air travel. You’ve got people on the no-fly list, unverified people in standard TSA, and if the US government likes you, you can get verified with TSA PreCheck. However the moment you do something dumb (say “accidentally” bring a gun) you get stripped of PreCheck.
Sure, but it was never said to be purely an identify verification process. There have always been other requirements, some of which were not public. It's fair to assume that one requirement is to adhere to the T&Cs, although I can't confirm that.
If Twitter thinks you're not morally good, they will actually remove a blue check which they previously granted (e.g. they did this to Milo Yiannopolous). If blue checks were just about identity veritifaction, this would make no sense since Milo's account's identity was never in question. Ergo, it isn't just about identity verification.