Not only was Sketchfab more for hobbyists, Sketchfab catered to a more general audience, not just game developers. Sketchfab had models for archviz artists, general rendering artists, look development artists, hobbyists, and models for all different 3d softwares. Not only was sketchfab a place to sell models, it was a place to showcase models. While Unreal marketplace was mostly a marketplace catered towards game developers as it worked closely with unreal engine.
I know for a fact, none of the models I create are designed for game developers. I create all my models for rendering/marketing purposes. Combining these two very different marketplaces was a horrible idea. Game devs dont want non-game ready copyrighted content in their marketplace, and the rest of 3d artists dont want purely game ready copyright free content. So how are they going to resolve this? Someone is going to have to compromise.
Coming from the more rendering based side of things, I like having branded models on these marketplaces. What rendering artist wants to put a render of some non-distinct copyright free car model on their portfolio? I understand the issue with commercial usage of these types of models, but this is why we have licenses dedicated to non-commercial usage. Not everyone who uses Fab is a game dev.