For 4k video, H.265/VP9 have better quality at the same file size, or equivalent quality at a smaller file size. Bandwidth usage matters for keeping people on the website and viewing Google's ads, so Google wants to ensure a smooth streaming experience. And since YT compresses so heavily, uploading in 4k increase the quality at lower resolutions (it seems weird but a 4k video uploaded to YT and streamed at 1080p will look better than a 1080p video uploaded to YT and streamed at 1080p).

And as xnyan said, high motion video really destroys video quality on YT, this video really shows how bad it can get: https://youtu.be/r6Rp-uo6HmI