Appeal to Developers
There are a lot of developers in the incremental games community - the genre seems to draw them in, and convert a lot of players into developers. Let's explore the reasons why this genre appeals to developers.
Incrementals are Easy to Make
Compared to other genres, incrementals have quite low expectations. You don't need to make fancy art, or music, or lay things out nicely. If you can make a button and learn the few lines of code necessary to make a number go up, you can make an incremental. This low threshold makes the genre perfect for those who are actively learning to code and haven't developed any gamedev-related skills yet.
Additionally, unlike other genres incrementals are uniquely easy to implement in a normal web page - no need to worry about rendering sprites, moving them around, implementing physics, etc. New developers can just use HTML to add a button, and the game is now available in your browser. You don't need to choose an engine, have admin privileges, or hell for the dedicated you don't even need a computer - there are tools for web development that run in the browser itself, so you can technically use your phone if that's all you have.
Javascript is a perfectly viable language for making web games, whereas other genres are typically going to require using other more difficult languages to learn. There are countless javascript tutorials that start from 0 knowledge of programming, making it incredibly accessible to beginners.
Players are Easy to Find
Once you've finished your game and uploaded it on github pages or itch or just copied the link if you're using glitch or replit (all of which are easy to do), anyone can now play the game in their browser. This low barrier to entry has shown tremendous success in getting completely unknown developers to have thousands of plays.
The incremental games community, which mostly centers around r/incremental_games, is always looking for new games and tends to flood any new ones posted with initial players.
Having your games be played can be incredibly motivating, and the community makes it quite clear that you can expect players to play your game. These communities - both for incremental games in general as well as game-specific communities - tend to be very developer friendly as well. A lot of the developers know each other, and welcome new developers with open arms, often with dedicated channels for programming help and discussions.
Monetization
I'd like to clarify that everything I've said above mainly applies to web-based incrementals. Incremental games are also incredibly popular on mobile, but with a much different culture and community. Many mobile gamers will still participate in the web-focused community for the culture. This web-focused community has a culture that has been criticized for being "anti-monetization". Ads, IAPs, and similar forms of monetization are often criticized, mainly due to the abundance of completely non-monetized games available from hobbyist developers. There are exceptions, like paid games often being considered fine, like Increlution or Stuck in Time, or donation ware games like kittens game, but even popular games that have IAP see some level of regular criticism, like NGU Idle, Idle Skilling, or Idle Pins. A large part of this can be explained by the community being hyper-aware of the addictive nature of this genre and its susceptibility to exploiting players.
On mobile, however, monetization is the norm and expected. If an incremental game is available on mobile, it almost certainly will be monetized, and mobile players are aware and accepting of that. Mobile incremental games, due to their addictive nature, tend to make a lot of money. It's very lucrative, and therefore these games are quite abundant on mobile storefronts.