For me, these are the main reasons:

-They understood pacing better. Even beyond having a sense of brevity and wit, but a real grasp on how long certain scenes should be cut, where escalation should begin and lead, and the ability to say when certain scenes require quietness or other emotions.
-They felt more inclined to do whatever they felt was entertaining. Today, it's expected that comedy slapstick animation must have TNT, anvils, and the other tropes. Back then there was less expectation to constantly use the stereotypes, because they invented the stereotypes in the first place. There is value in thinking of new premises, jokes, executions, etc. A lot of shorts that their own feeling.
-Characters used to be more dramatic. They would think, fear, envy, be greedy, and therefore touch upon the faults of human nature. There was an internal cognition, that the characters are thinking about stuff, reasoning, or acting on impulse. Often the entertainment was a vehicle for stuff like this.
-Yeah, of course the humor was more politically incorrect. Obviously I'm not saying we need nigger and jap jokes to come back, but there is a fine ingredient in having smoking, alcohol, ladies, guns, violence that actually hurts, etc, that is missing when the formula becomes sanitized
-The sound and music was better. Not much to say beyond that..