How odd. The entry actually made me more excited to include one in my adventures. Heck, I even dug out Night Below for inspiration.
I wager its to mimic the "immortal alien horror" concept; they long ago mastered a fail-safe that allows them to cheat death by being reborn (take that Death God!) in a hospitable environment far away from its killers.
You don't kill aboleths, you just inconvenience them.
And why the plane of water? They are not water elementals or anything natural. They are aberrations! They aren't even fish! An aberration appearing on an elemental plane would draw the wrath of every elemental there.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.