I agree with Linus from LTT where he said it's because the TV and Phone industry have effectively ignored 1440p as a resolution. Movies are either HD (1080p) or 4k and so TV and phone panels are made in those resolutions. Monitors may be 1440p (which I find most useful) but only native PC games actually output at that resolution.

This really goes to show that Stadia is a video streaming platform (i.e. something to help compete with Twitch) rather than a game playing platform.