It's probably more that HTML (or a specific pre-built template or engine which happens to be coded in HTML) lends itself more readily to menu-web type sandbox setups and thus any clueless novice wanting to make such a sandbox gravitates towards HTML. The ones I've seen are
relatively sparse on accompanying images and the text flow tends to be
awful for dialogue so I imagine anyone wanting to make something closer to a linear visual novel will do the smart thing and use Ren'Py instead.
Then of course, being clueless novices, and often spitting out some v0.02 barebones demo, the product which ends up on here tends to be the barren skeleton of a sandbox with very little content other than 1 or 2 linear storylines. Doesn't mean they were intended to remain that way if the devs got around to actually finishing them.
A lot of the "life sim" styled ones also seem to be of Russian/ex-Soviet origin which leads me to believe a certain style of gameplay might be much more popular over there compared to the West or Japan. I haven't seen many "public molestation" games from outside Japan or "furry kitchen sink" projects detached from Western sexual liberation either.
Mind you I have also seen other HTML gameplay formats including 2D tile RPGs (
Rogschard comes to mind) and less "sandboxy"/more "constrained text-based adventure" styled games such as
Transylvania. I have also seen a plethora of empty and/or essentially linear Japanese RPGM-based games, and both RPGM and Ren'Py provoke a lot more "skip, SKIP, SKIIIIIIIIPPP!!!" reactions via dozens if not hundreds of "dialogue slides" full of cringy sex sounds or dirty talk which leads me to believe the feelings of emptiness in some HTML games might be due to the lack of an autoskip/fast forward option or a refusal to pad them with inane filler (or using RNG instead, ugh).