Besides the obvious reason of it being a brand new feature of the installer, one reason may be the "hassle" that full-disk encryption comes with. FDE isn't just

Besides the obvious reason of it being a brand new feature of the installer, one reason may be the "hassle" that full-disk encryption comes with. FDE isn't just "home directory encryption", which unlocks automatically when you log in. It requires manual password entry (or key-disk provisioning) in order to init and load the kernel at all.