Because bonnet means “any of various hoods, covers, or protective devices”.

The etymology of which:

bonnet(n.) early 15c., “kind of cap or bonnet worn by men and women,” from Old French bonet, short for chapel de bonet, a cap made from bonet “kind of cloth used as a headdress” (12c., Modern French bonnet), from Medieval Latin bonitum, bonetum “material for hats,” which is perhaps a shortening of Late Latin abonnis “a kind of cap” (7c.), which is perhaps from a Germanic source. (If that is correct, a chapel de bonet would be etymologically a “cap made of cap”).

So bonnet as a cover for the forward part of a vehicle originated with bonnet as a cover for one’s head.