-Key antagonist and ally of Thor, the central villain of Avengers 1
-Murdered by Thanos, and a key motivation for Thor's power increase
-Snatched the Tesseract in Endgame, setting off this series

Not to mention he's arguably the GOAT comic book movie villain (turned part-time anti-hero), and I only say arguably to be diplomatic. I don't think we've seen another villain this well executed over so many films.

Winter Soldier would be a very close second, but he's been a full blown hero outside of TWS.

He's one of the most significant figures in their entire film universe, and giving him his own show with a second season on deck before the first hits is a no-brainer.