The game is no where near complete/done yet. If they put it out on Steam, it will reach a way larger audience. But that's a bad thing! People may juts put reviews out that the game is bugged, unfinished, cash grab etc. They stated somewhere before (maybe on the Discord) that this Epic exclusivity both helps them pay the bills a lot easier, so help in keeping enough people to work on the game to make it better (obvious), but mostly that it helps them to have a smaller audience to playtest and provide feedback. They benefit a lot from the current structure.

Also, they have not stated anything regarding publishing it on Steam, but if you read between the lines and look up previous deals, it is extremely likely that either at launch, or 1 year after launch, they can publish it on Steam.