UHS-II and UHS-III are, as you point out, quite a bit pricier, as are other options other than UFS Card. M.2 storage has a power consumption and size problem, so I leave that out.
The main impediment for external card storage is, as
@Pokemaniac suggested earlier, Nintendo putting in the legwork to establish the market for it that does not yet exist, no matter which microSD UHS-I alternative they choose. That is doable, especially for UFS for the reasons previously mentioned, and they absolutely should choose the option that is cheapest to the consumer, but it operates under an assumption.
That assumption being that, if internal storage and Game Card read speeds increase as we expect they may need to, Nintendo will want to maintain the ability for loading games off external storage and find a solution that grants them read speed parity. But they could just as likely revert to the idea of external storage being storage ONLY and require games to be played off internal storage only, bringing us back to the days of the Wii. Such a decision will ultimately come down to how much internal storage capacity they have, so as to prevent game juggling across storage when possible.