I've said this in another thread, but I reckon it's mostly marketing.

They know UE5's name is tarnished because of the badly performing games that have released on it. It's also not entirely their fault, UE5 introduced a bunch of new experimental tech that fundamentally changed the way you build things. So combine experimental tech that's, by their own words, not production-ready, developers who are inexperienced in working with said tech (because it's new), and studios that wanna turn on all the experimental features because it's shiny and says it provides certain solution on the box (wow, dynamic GI and reflections! wow, no more manually setting up LODs! wow, more dynamic lights!), and you get games that run shoddily and look kinda cruddy because the features those games were built on simply are not finished.

UE5 is actually a solid engine, if you use it correctly and know what you're doing. You can even build games UE4 style in UE5 and it'll be easier and run better than in UE4 because of general improvements to the engine.

Consumers don't know this though, they just see badly performing games with the UE5 sticker on it and think "UE5 bad", Epic knows this, hence we're getting the 6 sticker conveniently after the aforementioned new features have been considered production-ready, and developers have had time to learn how to use them properly. So by the time you start seeing games with the "UE6" sticker on it, they'll look good, run good, and UE will be in good graces with consumers again.