Because they can not eat anything else.
The (effective) generation of a vampire has a significant effect on his level of power in a variety of ways. This includes the maximum amount of vitae that they can maintain, the amount of vitae they can spend in a single round and the maximum value that their attributes can reach.
However, the lower (more powerful) generation has one drawback: it makes you a more demanding eater. While a typical vampire player character can subsist on almost any blood in which they can get their fangs, older vampires can only feed on human blood. For the older vampires, even the fresh blood of the healthiest of the species is not pure / refined enough; they can only feed on other vampires.
Of course, each time a vampire feeds on another, they risk forming a blood bond, giving the vampire power to feed on them. This risk can be easily mitigated by draining the "donor" dry; you can not join a corpse (not a relative). So, a vampire that should the feeding of other vampires will prefer to kill those on whom it feeds, even if this diablerie does not grant any other benefit because the victim is weaker.
The antediluvians are obviously old enough to have this type of feeding restriction. In addition, their blood reserves are probably huge (or even infinite) in capacity; Once they wake up and begin to drain their progeny, they are probably not satiated before they drain all the children they can find.
(I am away from home for the holidays, I will add references when I return home in a few days and have access to my books).