In my eyes, part of the reason for this is that they forgot a key element of penetrating a market... you need a potential customer base that is actually displeased with the current available solutions and is actually looking for an alternative. And, by and large, the current storefronts had done a good enough work of pleasing their customer base that, when the Epic Store rolled out, few people were actively looking for a switch, to the point that no bonuses or goodies or exclusives that Epic offered could outweight the friction of moving from a platform that was perfectly serviceable, please and thank you.
The whole thing was just mistimed. They should have waited to see if Steam committed some sort of fuck up. They should have waited for some type of negative sentiment. I don't know. I know that developers did feel displeased with some of the conditions on Steam, but Epic could only do so much to win them over with 88%'s and paid guarantees and what have you, when they couldn't offer them the most important thing: a paying customer base.