It's because Epic isn't bothering answering all the complaints. It's pointless. Most of them are related to parents supposedly not knowing that their kids charged 20 dollars a day on their cards for skins and such. A company like Epic is not worrying about their BBB rating.
Epic as a company is far better run overall. Certainly issues are there, but nothing compared to the hot mess EA is.
I don't really trust the BBB all that much as there is a lot more to how good/bad a company is other than complaints and response to those complaints.
As many have said, a lot of Epic Games' complaints stem from parents who are upset that their kid used their credit card to buy skins and they, somehow, blame Epic for it.
Then you have the ones who say "My kids account was banned and he claims he did nothing wrong, so obviously Epic is at fault". When most people know that companies don't just ban accounts for no reason (not saying it doesn't happen, but it's not exactly a common occurance).
Plus, they have 200ish complaints filed on that site against them. Yet there are literally 10s of millions of people who play Fortnite every month (obviously, they have other games, but Fortnite seems to be the game most complained about).
The BBB isn't all bad, obviously, but you have to take their ratings with a grain of salt. I've seen legit companies have absolutely horrible ratings and companies that are shady have A ratings.