Well...you've basically mentioned the similar games and you should already know WHY no one else even attempts it.
Seriously, this game is a massive dedication for relatively little gain. And people see that.
If Inui was "smart", he would most likely do a western-dev move - develop a decent, eye catching prototype and milk it on Patreon for years without anything of value being added for years. We are lucky he doesn't but it's a rarity these days.
Another angle is that people know they will always compete and be compared against both Inui and Kairi, so the bar is relatively high in terms of amount of content and work ethics (regular updates). If I were to finally go with my own miserable attempts at h-dev gamemaking, I'd NEVER try to do anything even remotely similar to HotM because I know the biggest compliment I'd ever get would be "oh, it's HotM clone"...which is not what one would want to hear about their work.