Switch’s operating system is based on the OS from 3DS. The ARM architecture was already well documented and emulated. Tegra has documentation from NVidia.
With all that, making a Switch emulator was relatively “easy”. They took Citra, the 3DS emulator, and worked from there.
Xbox 360 is a different beast. Even its OS was only kinda Windows, so they couldn’t just take Wine and a PowerPC emulator and call it a day. Taking long is IMO not much of a surprise because of that.