The distance between the start of the endgame to getting the two princesses ending is too large and involves a lot of choices along the way. The endgame is more like ACT 3 of the game than just an epilogue.
Strictly speaking, the endgame actually encompasses both the climax to Act 2 (your first confrontation with Selena) and the entirety of Act 3.
If you want to break it down, the game is structured like this:
Act 1: Everything up until the moment you are given free rein to explore the game after the tutorial.
Act 2: Everything leading up to the first confrontation with Selena.
Act 3: The finale.
Yeah, Act 2 is much, much bigger than the other two, but that's not unusual in video games.