One of Steam Machine's most attractive features at the time was the ability to allow PC gamers to play games in the living room. As time went on, Valve and Nvidia slowly began incorporating this same feature onto existing PCs. The demand for a dedicated Steam Machine PC quickly dropped and its high price of entry deterred even the most fervent of gamers.
https://www.notebookcheck.net/Valve-Steam-Machine-runs-out-of-steam-no-longer-on-Steam-Store-menu.294456.0.html