I think the goal is explicitly not to go all-in on normal web development, to maintain some opportunity to monetize.

D3.js started as/still is just a (collection of) JS libraries. Then Observable came along, and while it is a nice tool for tinkering with D3, it was not at all obvious how to then move your finished Observable D3 viz to a fully self-hosted, regular website (without paying a subscription to use their custom runtime or servers or whatever).

Now I guess they realize they strayed way too far from "the web" and are back-tracking.