what is it about raytraced animation that makes it more expensive to update the BVH mesh vertices every frame, compared to old-school bone animation where you send the info to the vertex shader every frame?

Massively simplifying here but when doing one rasterization pass you process a list of triangles, in order, once. When raytracing a camera view you query a database of triangles, at least once for each pixel. The database needs a fancy 3D spatial index for this to be fast enough. When you modify the list of triangles you have to fix up or rebuild the fancy spatial index too.

To make ray tracing practical, you need to update the ray-tracing accelerating structures when you animate.