Switch is weak. Weaker hardware is easier to emulate.

Umm....the 64 is weaker than the Switch...still has problems with emulation. Xbox 360 and PS3 are weaker than Switch....still difficult to emulate. I think more than anything, the fact that the Switch absorbed the Wii U's architecture and CEMU devs seemingly obtained knowledge that sped up CEMU development, that it also had impact on understanding of Switch emulation.

I think it's kinda all of the above. Weaker hardware is easier to emulate in terms of churning out an experience that is somewhat performant on the today's PC. But system architecture also plays a big part in it in terms of how difficult it is for the people working on the emulator. If the Wii U's architecture lead to what the Switch is, that makes sense.

Additionally, the PS3's architecture is so convoluted, it probably never will be emulated (not even by Sony for backwards compatibility).

As for 360, I am sure emulation wouldn't be too difficult, but there's an entire other reason people emulate consoles: general interest. Most people are interested in playing Nintendo console games generations later compared to most other companies, although Sony and Sega have mass appeal, too. I don't think many games on the 360 elicit that type of interest in the people putting the effort to emulate the system, so it's a slower process. Aside from a few exclusives, you can find most of those games on PC.

The problems with the 360 and PS3 emulators had little to do with the emulation of the CPU but a lot to do with the emulation of the GPU's modern features with respect to the common graphics API's at the time. CEMU wasn't that far ahead until BoTW released and there was demand (and donations) to develop it. Likewise with this emulator.

The pace is more of a demand thing than a supply one. If people threw ten thousand dollars at the Xenia or RPCS3 teams, we'd see a lot of progress there too.

I still want to play Lost Odyssey on PC though.

It's pretty natural for there to be less demand for emulators for Sony and especially Microsoft systems. Looking back at the 360 library, what games were there that didn't come out on the PC as well? Halo 3, Gears 2 and 3, Forza, Bad Company 1, Dead Rising 1? There are a few, but games like Mass Effect, Bioshock, COD, Battlefield 3, Skyrim, GTA, Gears 1 and Dead Rising 2, Witcher 2, Arkham City, Portal, Left 4 Dead, Assassin's Creed, ect. were all on PC and often better on PC than on any console. Why emulate when you can just get a superior version of most of the console's best games on Steam or Origin? With Nintendo and Sony you know you can't play Mario, Zelda, Uncharted, Smash, Ratchet and Clank, Mario Kart, Metroid, or Killzone on PC, to same nothing of the huge number of other exclusive series they have.

More people will be using this to steal and play Nintendo games for free, than people who legitimately own a Switch + games, but just wanted a higher resolution on PC. For decades Nintendo's core strategy for selling consoles has been to rely on the strength of their in house games. Now that Yuzu works, there's no reason for many people to even bother getting a Switch.

This is something for people who never wanted a Switch in the first place. Emulation doesn't really affect sales much I think. Otherwise no one would buy a NES or SNES classic.

People play it because its free, not because they are fans.

And the piracy begins

You can say that to every Nintendo portable and yet, their sales are always higher than the competitors quite a lot

Then you don't really want to play them it seems.