Because they can't eat anything else.
A vampire's (effective) generation has a significant effect on their power level in a variety of ways. This includes the maximum amount of vitae they can hold, the amount of vitae they can spend in a single round, and the maximum value their attributes can reach.
Lower (more powerful) generation has a drawback, however: it makes you a pickier eater. While a typical vampire player character can subsist on pretty much any blood they can get their fangs into, older vampires can only feed on human blood. For the oldest vampires even fresh blood from the healthiest of kine isn't pure/refined enough; they can only feed from other vampires.
Of course, any time one vampire feeds from another they risk forming a blood bond, giving the vampire they fed from power over them. This risk is most easily mitigated by draining the "donor" dry; you can't be blood bound to an un-undead corpse. Thus, a vampire that must feed from other vampires will prefer to kill those it feeds from even if this diablerie grants it no other benefits due to the victim being weaker.
The Antediluvians are obviously old enough to have this sort of feeding restriction. Further, their blood pools are likely enormous (or even infinite!) in capacity; once they wake and start draining their progeny, they're not likely to be satiated short of draining every single childer they can locate.
(I'm away from home for the holidays; I will add references when I return home in a few days and have access to my books.)