[numpad0]

It's a bit perplexing, but 1st gen VTubers were all 3D. They evolved through natural selection into current 2D forms, only slowly growing back supplemental 3D forms.

My guess as to why is that full 3D must have been extraneous cognitive load to viewers - xkcd wouldn't have been as popular as it is now if it had been drawn somehow by Rembrandt himself. It owes its success to Randall Munroe's minimalist art style. That kinds of things.

[dotnet00]

It's probably because high quality 3d is just much more expensive to do. You can easily do all sorts of effects in the constrained environment of a 2d model that take a lot more skill to pull off in 3d.

Plus, ultimately, the anime aesthetic is a 2d thing. It's a lot harder to make a 3d model look good in a 2d art style, as a ton of anime over the years have shown.

[numpad0]

A lot of them had 3D models and quit using it. So I doubt it had to do with the cost of making one. IMO the second one is much more likely.