abd is a really simple mod with a simple way to survive, therefore it is really easy to understand and pick up. betting server hosts started using it for their servers because of that, so that most people could play (while still keeping a decent mod for the skilled players to use)
the fact that abd is easy to pick up and not as easy to master makes it the perfect mod for the new players to start playing at a competitive level
since abd was adopted by so many people, techniques are always being developed, always new tricks to do whatever, etc. it wasnt difficult to predict that non-competitive abd would adapt from competitive abd, because the competitive abd scene is huge