Eh, I think it's primarily because of two developments:
One the one hand, we have the effects of Renpy, Patreon and the proliferation of accessible 3D Rendering/Posting software::
-Renpy allows for a much easier creation of visual novels as opposed to creating them from scratch, or programming an arcade game. Renpy scripting can be as complex as you want it to be, but for a beginner it's really easy to get started and have something fun (there were open VN engines before - Kirikiri and NScripter - however they were in japanese.)
-3D Rendering/Posing removes the second barrier to entry - skill with drawing. With a 2D, gameplayfocused game, you either needed to be able to draw yourself acceptably well or needed someone else who could. With a poser, that barrier to entry is much lower (as in, this art is functional and I can show it to people without being ashamed) - and it's a new, fun skill to learn, instead of something you hated since you had to do it in elementary school...
-Patreon both offers a way to distribute and monetize the game, and allowed for games to be much more widely played while in development. Which not only makes creating and publishing erotic games much more attractive, but also meant that now that game you were tinkering on and off again, but had only shown to maybe a friend or to before? People are now playing it.
The combined effect of which has lead to a downright insane boom in english-language erotic VNs, in large part by people who previously wouldn't have created any erotic game at all - people who had an idea, a concept/scenario, and maybe the wish to create a game before, but no real idea of where to start. It's hard to measure, but we're probably seeing more activity in a week than what happened in a year erotic game-wise way back when Newgrounds and AIF groups were the only game in town.
On the other hand, gaming changed. Not only did visual novels shift from being a strange type of "is this really a game" genre only popular among "weird otakus (who probably learned japanese)" to something that's actually quite popular and mainstream, but tastes and expectations changed to - story and characters are much more valued (as opposed to gameplay), arcade games went from something everyone had played at some time to retro games, being replaced by FPS and action-adventures.
So you have 1. a great flood of visual novels, 2. some of the people that want to make gameplay-focused erotic games going for 3d games (another change in gaming -"modern" (AAA-inspired) game development requires much more work hours, so the failure rate is higher and updates/games fewer), and 3. a few others that still make some arcade games, but compared to the first two they're much less visible. The couple of jump'n'runs, the puzzle game here or there - unless you're specifically looking for them, they'll just fade into the background.
(For me, the thing I miss about those days are adult interactive fiction games/gamebooks. Sure, VNs are similar, there's chyoa and fiction.live, and there are some twine games here. And in reality we probably also produce more of those nowadays than back then, but I still feel like there should be so much more out there and I'm just not aware
)
