>When Red is encountered in Mt. Silver during Generation II, he has no speech, merely repeating an ellipsis before beginning the battle.
>This is a reference to the fact that during Generation I he is a silent protagonist, answering only "yes" or "no" questions. He also remains silent when confronted in Generation IV.
Source: Bulbapedia/ Red
This is what Bulbapedia says. It could also just be dramatic effect, being one of the strongest NPCs in the franchise, if not the strongest.
Note: In FireRed and LeafGreen, Blue sarcastically calls him a chatty gossip.