01. The storefront is actually functionally super limited. You cannot add more than one game to your cart at a time, so to buy games on EPIC you have to buy them one at a time, even during sales. If you buy too many games at once, your account gets flagged and you have to work it out through their systems before you can buy/play anymore (multiple users have experienced this during the last sale). Developers have no ways to remove their games from sales except to remove the games from the store entirely during the time of the sale. There is no set regional pricing, so game prices in different regions are either SUPER expensive for other countries or super cheap, and sales are broken in these regions. The actual store is limited in functionality at best, and actually broken at worst.
02. Steam is the standard, and Valve and Steam don't enforce exclusivity to their store, games could be sold on both Steam and EPIC for developers to make players have a choice in what they want based on their services. The only thing EPIC offers is exclusivity for the end user, the developer thing is nice but them trying to bring audiences with that is not going to go well in a platform that prides itself in it's open ended-ness and with the existence of better open options than Steam already, such as GoG or itch.io.
03. PC users are tired of too many launchers. Imagine if when you launched your console, you had to then launch five different programs at once, or separate ones to access different parts of your game library. Epic came on the trail-end of the tiredness of publishers trying this.
04.Epic has a shoddy at best history with PC Gaming, they said some pretty ignorant things about the scene some years ago, and then their return to the platform show their lack of understanding of the scene, what PC Gamers care about, and trying to monopolize for that reason to try and compete and make the PC space more limited rather than open in options. Steam solved the problems of piracy in the PC space by providing a service that was more beneficial than pirating due to increased ease and features for the user. They became so big in the scene due to what their "over-excessive DRM" provided to the end-user. Epic literally provides nothing to the end user except the games that are exclusive to it, it's the only way it competes.
05. Epic isn't really helping the people who need it. All of the games they've picked up were from developers that were already successful in other places, IE "proven" devs. It makes sense to help these guys, but the actual quality struggling games that have no attention but are actually good won't get picked up until they succeed on Steam. Epic's store literally needs Steam to work through their curation methods, because a start-up indie studio has no chance to get onto the Epic Store until they've "proven" themselves elsewhere. Epic as a store literally can only work due to how they curate by trying to buy out people who've already found success elsewhere, which makes them like a vampire storefront. It also creates resentment from people who made a game a success elsewhere on a storefront then have to switch storefronts for the follow-up, especially when said storefront doesn't provide anything really outside of taking up space.
There's more, but a few off the top of my head.