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.
Just to confirm - Metro Exodus pre-load for existing Steam pre-order customers WILL HAPPEN, and we will announce expected start time in due course.February 12, 2019
We regret to inform that pre-load via the Epic Games Store is not planned.February 12, 2019
"Preloading" is simply the process of downloading and installing a game ahead of its release date, so you can jump in and start playing the moment it goes live. For people with slow or unstable internet connections, being able to preload a big game like Metro—the download clocks in at around 50GB—can mean the difference between playing on launch day and playing a day (or two) after.
Deep Silver did not say why preloading won't be available through the Epic Games Store, but a rep confirmed that the tweets are accurate. We'll have a review of Metro Exodus this week.