Each new custom terrain has its own sets of objects, and each object that comes with the terrain has its own shape. It can be a big shape, a complex shape, or anything in between. But whatever or depending on the shape of the object, it can present a new obstacle to overcome. To overcome a new obstacle could take time, because you have to remember every details that requires to maneuver around it, and not to mention how you can use it with your weapons and tools. I have only mentioned object, not ''objects'', and every custom terrains has a lot of objects. Which could mean you have to spend a lot of time to learn all these Intricacies specified here. Some people may not choose to spend their time to ''master'' every objects of every custom terrains. And WA has over 60 terrains as far as I am aware of.
Though In hypothetical, I could see an argument against this could be ''If you are already an experienced WA player, you should be quite knowledgeable of the pixel structure of the game, and every Intricacies that comes with it. And a new terrain does not require to be learned.'' Of course such argument is flawed IMO. This is because of the esport players of Counter Strike 2. They must learn new maps when it becomes available for them.
But this is just conjecture of course. I personally don't like a lot of the custom terrains because they are not aesthetical pleasing to look at. Super Mario Bros 3 (brown) terrain, is quite pleasing to view and play on, contra to the Cosmic terrain. Cosmic is pretty, but noisy to play on.
If you are really passionate about custom terrains, make a new topic where people could drop their favorite terrains, and sometime later make a poll with the nominated terrains submitted by the user.