Because emulators are entirely legal and doesn't facilitate piracy on their own

Combination of anti large company sentiment + people feeling entitled to get things for free if I had to guess. It also usually feels wrong when a corporation threatens a lawsuit over a single person since the US court system heavily favors the person with more money and it's probably a true statement to say that Nintendo has more resources than the lead dev.

Modern Vintage Gamer on YouTube had an interesting take in that by stifling emulator development now it will hurt the industry in the long run because Switch exclusives will become increasingly difficult to play once support ends (an argument I myself don't find all that compelling)

Nerrel on YouTube has a well put together and researched video on emulation where at least in the US it's been tested in court several times that emulators are legal, but obtaining the code for the emulators to run is almost always not since you usually have to make a copy and that violates the publisher's right to copy