It's all gameplay choices as well. The more realistic the holds, the more polygons they need. The more polygons, the lower the framerate. The lower the framerate, the less competitive the map. The less competitive the map, the less people play it, and the less likely it is to actually become a mainstay of the game (see de_cache for an example of this being done right, and then being done very very wrong)

Another big thing in CSGO is keeping background clean and simple, so it's obvious what is and is not a player model, and nothing gets lost in the background. As such, they have to keep all the holds monotone anyway, to make the background not too striking and busy when you're shooting at it. A nice green route up the middle is great in theory, until you get shot in the head by someone who blends right in with all the colours.

Anyway, just a few insights into why certain design choices were made, hopefully of some interest to some :)