a map needs to emphasize something about a team's skill.
Process emphasizes individual DM, team DM(synergy), and the ability to manage momentum(knowing how far to carry your momentum and how to stop a roll), making it the perfect pug map.
Granary and Badlands, for example, had a role in emphasizing tight teamplay and in general had value in requiring a team to know precisely when, where, how, and why to take fights, meaning their value as pug maps is nearly nonexistent, but they were great ways to stress a team in ways that exist far less in other maps, probably because the skills that allow a team to dominate Granary and Badlands aren't glorified in the way the skills required to dominate Process are, for example.
Gullywash does a good job of highlighting your splash classes without actually leaving scouts behind by letting you bomb to/from anywhere and having reasonable high ground without such insane high ground that scouts can be made irrelevant.
All of the long-term 6s map pool has some sort of actual purpose in enabling different types of play.
As an example of a map that is hated and for good reason, Clearcut exists to constrain a team and is a painful slog of a map because of it. The left side of the map exists to be a death zone, constrain the speed of a soldier's rollout, and to arbitrarily prevent players from pushing out of or into that portion of the map. The right side of the map exists to be a giant spam corridor which is also arbitrarily segmented off by an invisible wall blocking you from high bombing into the back portion of the map, for example. I'm sure other class-main player would be able to explain how the map constrains them, but I've at least provided what I saw a soldier.(iirc our scouts were complaining about point being a low ground death hellscape and being permanently on shed duty because there is no other option than to watch the only two bombs on the entire mid and our demoman had to essentially afk holding m1 looking at sheds 24/7)
And as an example of a map that people find hard to accept, likely due to its redundancy, Propaganda. Propaganda is basically Process 2, electric boogaloo. Mid can be turned around by any class given they play out of their fucking minds and it is about equally possible to get rolled to last equally fast. I would say I like the soldier routes on Propaganda more and the pushes into and out of last are more interesting than on Process and that the second is generally more scrappy, but it still doesn't really fit an entirely unique role and the skills required to dominate Propaganda are nearly the same as the ones required to dominate Process, so it's hard to accept a map so redundant. Personally, I like Propaganda and understand why a lot of people thoroughly feel no need to waste our collective time with the map, and don't see why it exists in the same map pool as Process.
And there are things that break maps regardless of what potential exists.
Badlands mid is known specifically for being a hellscape made specifically for the purpose of torturing the souls of anyone who believes a splash weapon should function and Spire is nearly as bad. Its last also caps too fast.
Propaganda b16 crashes half of at least one team connection almost every time I have ever played it, has performance issues, and also was incredibly visually buggy on some computers(like not being able to see through thresholds, for example) on older versions.
Clearcut has intermittent performance issues.
Granary 2nd has a bunch of dumb bullshit in spite of the main arena of the point itself actually being an interesting use of thresholds. Lunchbox exists. Why?