For me personally as a non-coder, I can grok Inform 7 much more easily than I can the pure code of TADS and Inform 6. As mentioned, I think the author-base swells when writers feel more confident with the more-readable “almost English” syntax instead of learning a completely new style of code-formatting.
With Twine, I believe it’s more about presentation than syntax and pure “grammar” of parser engines. Putting out a game that displays in the default format (bare Harlowe/Jonah/Sugarcane) is equivalent to leaving default parser descriptions “As good-looking as ever.” so more experienced authors do a lot to make it not look basic. There are Twine games made which don’t feel anything like Twine. There’s not a library per-se, but story formats which change the coding and utility of the language, even though they are still Twine - similar to Inform 7 extensions.
I disagree that there are “less VNs” made overall - I think the VN audience/authors are a niche we don’t hear from here much about since VN is it’s own special game type and there are more dedicated forums and social spaces for them. The audience doesn’t overlap into parser or choice games as much. Ren’py is still used, and the thing about VN is the gameplay and formatting is pretty much set and all the engine needs to do is do that well. Major game companies likely have their own proprietary engines to accomplish presentational sugar - Danganronpa, for example, is pure Visual Novel for dialogue but with a 2.5D map and point and click environment bolted in.