Personally I think it is because it has terrible performance. Scenes that Unreal seems to be able to render at full frame rates run at a fraction of that in Unity. Even its vaunted 2D system is slow as balls... load times for Graveyard Keeper, as an example, are agonizingly long. And for what? A hundred megabytes at most of 2D pixel-art assets? What is the engine doing with all that time? Certainly not just loading and decompressing shit into RAM.