East can see plenty of tricks for the declarer in diamonds, but he controls the suit with the ace. If East does not do something, when the declarer regains the lead, he can lead diamonds until East is forced to take the ace, while the declarer still has the ace of clubs as an entry.
East can see many tricks in diamonds for the declarer but controls the suit with the ace. If East does not act, when the declarer regains the lead, East should lead diamonds to force taking the ace, even though the declarer still holds the club ace as a potential entry.