Homebrew took off in a big way during the original xbox and contemporaries because they were powerful devices you could connect to your TV and use with a nice controller. At this point it was hard to connect your PC to your TV (VGA ports were not necessarily common until the 360 era, and computers having svideo out, never mind composite or RF, was a rarity) and controllers were awful or needed adapters.
The GBA, DS, PSP and such also represented some best in class devices for portable computing at the time. Technically you had a few semi open source phones in Europe that you could write code for (contracts probably involving surrender of a testicle and your firstborn), and some palm computers but just no. The DS and PSP on the other hand were network enabled devices that ran real code and thus could be used to do things, or possibly hack things as it is just a kid's toy).
Since then we got the likes of the raspberry pi and android (and IOS) that you can use instead, or even write code for and make pretty good money doing. The rise of IOS (and android shortly after) pretty much killed the DS and PSP overnight (if gbadev or archives of pocketheaven are still up then go look for some of the leaving messages there, or future activities of some of the devs of the biggest pieces of GBA and DS homebrew). It is presumably also why the 3ds one was so mediocre in comparison to what came before.
I am not sure why the Wii got as popular as it did for homebrew (and for my money the xbox is still better) but I guess ubiquity, ease of hacking and cost were a factor there.
To that end better devices that are easier to code for with a larger install base (remember homebrew users = a smaller fraction of the already small hacked devices userbase) appeared and you could also make money on those (while we have since seen the rise of the monthly donation platforms it was not impossible before then, donation drives were a possibility and plenty of other means of sending funds existed that people capable of using your code could do, and by most accounts and what I saw it was absolutely tiny).
At the start of systems/hacked systems we typically get a tiny bit of activity as devs feel out a new system and have some fun but once that is done and it becomes more of a grind just like you are updating something for anything else (I can't imagine at this point the devs behind L4T for Switch are doing much that the devs for Linux Mint are not doing) the enthusiasm wanes.
I have not heard of Nintendo threatening homebrew devs. Custom firmware devs maybe (though that is rare and they usually spend their time going after installers, importers and the like instead), people making guides and people making videos too but straight homebrew is new. The biggest threat there is usually either drama or some company wanting a project off the showroom floor as it were -- several hackers, homebrew authors, emulator devs and the like gained a contract to make a game or two and thus vanished from their homebrew stuff to do that.
