Have you even noticed that before? Well, most standard English playing cards used today derived from the original French models produced in the mid-16th century. For the purpose of mass production, the earliest cards were printed using woodblocks.
Disfiguring occurred over the centuries as unskilled block makers distorted the original designs, resulting in hands, symbols of office and other attributes losing their meaning. Among the many distortions that took effect, the King of Hearts not only lost his moustache, but the axe he was originally holding became a sword.
Diamonds, clubs and spades are by association linked respectively with the corruption of wealth, war and death. In contrast, the heart as an organ is pure, open, undisguised – it does not wear artifice – hence the clean-shaven King of Hearts.