Someone should make an "Epic Store problem FAQ" that we can link to every time someone comes into the thread and doesn't understand the problem.
First we have what is basically an unwillingness to support the whole thing Epic is doing. The whole idea of exclusivity deals, and especially when the games are already up on the Steam store but get yanked off from there. And Metro is the absolute worst one, since it was even up for pre-orders before the sudden switch so very close to launch.
It's not the same as announcing an upcoming game (that did not have a "shelf space" on Steam) as a store exclusive. This is more like going into a game store, pulling the game out of someone's hands and putting it on the shelf of another store with other rules.
Then we have the security problem. Yet another company you have to trust with sensitive information, and this company in particular is a huge target (due to Fortnite) and has had some huge security issues before. It's not just "installing a launcher", you have to actually buy the game too.
And then we come to the availability issues, not even being able to buy it in certain regions and not being able to use the store at all if you use Linux (an OS that Steam is supporting a lot from what I understand). In some regions you might still be able to buy it, but it is now more expensive than it was before. Some others got really good prices tho and no one really know where this will end because Epic is apparently scrambling to implement some regional pricing. But for now the pricing can be a big problem depending on where you live.
Another availability issue seems to be lack of an offline mode. To launch some games or even see your library, you and the Epic servers have to be online.
The lack of forums and reviews is a big point. People are going to the Steam forums to get help with problems in games you cannot buy on Steam (yet).
And then we come to the features, or rather the lack of them.
Some refuse to buy games without achievements, some couldn't care less. Some use the community features a whole lot (friend activity, sharing of screenshots and such) while other basically don't even know of their existence.
Controller support, cloud saves, modding. There are a bunch of stuff that are more or less important to people. The one I do not want to let go of if I have any kind of choice is Family Sharing.
Do I buy games on other stores and use other launchers? Yes, for first party exclusives that I really want. No they don't have the features I want but I can take that for these games. The prices are great (thanks to an Origin sale I just got the newest Sims 4 expansion for just over 10€) and I trust these companies.
Do I trust Epic? Nope, neither with keeping my information safe nor their ability to turn their store into something that has even the basic of features. Do I want to support their anti-consumer way of doing things? No.
I am not a Steam-only person (altho I am close to being that), I just don't want to support Epic at all.