The reason developers are going with Epic is due to cost. Steam takes 30%+ of all sales through the Steam store. That's a lot of overhead the game developers have to overcome or compensate for. Epic is taking around 10%, so it is easy to get developers to try them out. This price point is part of why Bethesda created their own store/launcher/distribution for Fallout 76, instead of using Steam.
Steam is also turning out to be a bottleneck. Steam has ~90 million subscribers. Epic has more than 250 million off of one damn game. The number of eyeballs/impressions for developers is far, far greater than Steam, and at less than half the cost.
The Epic store is new, and the demographics of the Epic userbase may favor certain games, but expect to see a LOT more games launch on Epic and not Steam. And yes, expect to start having multiple game site accounts. Steam, Epic, GOG, Origin, etc.