My guess is connectivity. I was only happy with the Steam Link when it was using a wired connection. This isn't an issue in my home, but most homes don't have wired Ethernet all over.
I did not love the v1 Steam Controller, but it's easy to use with other controllers.
Based on https://store.steampowered.com/hwsurvey/ I would imagine out of 100 Steam players, most are playing at a Windows gaming computer with an Intel CPU, a bunch with Windows/AMD, then a few with a Mac, then one with a Steam Deck.
Most of those are probably NOT plugged into a TV, so in that way I agree that these are not typical Steam users. That's why the Steam Machine was developed, to bring gaming back to the couch in a way that the Steam Link didn't succeed at.
I did not love the v1 Steam Controller, but it's easy to use with other controllers.