when "Talkies" first came out, the audio capabilities of the time were horrible, and it made most actors sound terrible. If you watch movies from the 30's, you will notice how every woman sounds like a screeching banchi. They are very hard to listen to. Chaplin hated them because of this, and because he say the ability to talk as "cheating". Why have good acting to tell a story when the actors can just tell you what's going on? That's how he saw it. He thought they were lazy and cheap.
His main reason, though, was his famous character of the tramp. The tramp was basically like Mr. Bean. Mr. Bean, like the tramp, doesn't really talk, and if he did we wouldn't know what to do with it because we aren't used to it. He thought that having his iconic character suddenly talk would ruin him, so he said absolutely not. Chaplin had built his life on perfecting the silent film, and talking movies were basically making his art form obsolete. It's no wonder that he resisted them.