https://www.pcgamer.com/metro-exodus-will-preload-on-steam-but-not-the-epic-games-store/

Metro Exodus is exclusive to the Epic Games Store, unless you pre-purchased it on Steam prior to January 29, in which case you'll get it on Steam. It's a little strange, but probably the least-awkward way to handle the awkwardness of switching platforms just a few weeks before release. 

What's even stranger is that, according to the Metro Exodus Twitter account, people who get the game on Steam will be given a preload period, but those on Epic's store will not.