The Deck was available and at a decent price for years. But you're right, every one they made did get eventually get sold until the new and "improved price" versions came out.

Eh, I'd argue the steam machine isn't a PC to most people that don't already own a PC. Most adverts I've seen of the steam machine promote it very console-like, just with steam and more games. Everyone who reads enough into it, and knows enough about PCs to feel comfortable with Linux, most likely already has a comparable or better PC at home.

Because a PC enthusiast is more likely to build their own and it's too expensive for a non-enthusiast. The Steam Deck was as popular as it was because of the price.