Board Thread:Plants vs. Zombies 2: It's About Time/@comment-24725728-20140512192458/@comment-4009269-20140611162207

They probably made the Gargantuar level Night 10 because it's sort of like a finale boss level, so it makes sense to be the last level available in Part 1. The Endless Zone being in Part 2 is probably because not all the zombies have been encountered yet, so the Endless Zone would either have new unknown zombies, or be easier than it would be in the next update.