Well, a horse was a clear indication of social status; the entire institution of "chivalry" as a military force to be reckoned with is based on the fact that only a certain class of people could afford to ride: those who controlled all the land, power, etc. Tricking a gentleman out of this status symbol clearly demonstrated the canny sense of the socially "inferior" milkmaid/farmer's daughter/simple country lass, etc., over this scion of the ruling class: a theme inevitably delightful to the hearer.

Good tunes, too!