The Wii U doesn't have that much homebrew because it's the middle child. The Wii is a fun console to develop for because it's underpowered and nostalgic, and the Switch is better to develop for because it's the newest, fastest, and has lots more support from the bigger userbase.
-There's a barrier to entry for Wii U homebrew compared to the wii ($8 for a game to inject with CHBC unless that's changed) while modding the Wii is completely free
-The OG Switch is mostly free to mod (kinda costs $20 if you want an RCM loader) and it was able to be softmodded much earlier in its life cycle compared to the Wii U. The Switch was announced literally a week after the "Brazilian method" exploit was utilized to pirate games on the Wii U
Also don't forget that the 3DS modding scene was huge and blowing up while the Wii U was slowly dying out. All the devs were busy getting stuff running on the consoles that they all had
Not much is happening in the Switch homebrew scene within the last two years though. It started great and spawned some amazing apps. And obviously Atmosphère as a CFW is outstanding. But it's not that we see new homebrew apps for the Switch every week although it's easier to develop for it, compared to the Wii U.
I don't know what the exact reason is for that, especially since basically
all consoles (over 100 million) are hackable through a mod chip and around 20 million (all consoles up until June 2018) are even hackable via softmod. 20 million! That's even more than all ever sold Wii U consoles on this planet.
I would have loved to see a real browser like Chromium or Firefox on the Switch. Or something like KODI or PLEX (NXMP is great though).
I think a lot of that has to do with all the unnecessary (!!!) drama that happened. Talented people like
@rajkosto @JRoad @HiD @m4xw @XorTroll and even
@blawar (say about him what you want, his talent was undeniable though) either left the scene for good or at least drastically decreased activity.
They are not to be blamed. m4xw for example might have decreased his output since he as working on RetroArch and it is basically done for the most part as it is now in sync with the official releases and thus gets updated along with them. XorTroll is rather active on his Discord than here and I think blawar still updates Tinfoil silently from time to time.
I still feel like these people
could do so many more homebrews if they wanted (and had the time?).