Probably because its a convertible. Convertibles are usually a bit heavier even when compared to the same model car with the drop top. The car needs more solid parts because the car isn't as whole or sturdy as a regular car since half of it is literally made without a roof.
Plus lets not forget the S2000 was made after most Integras. People like to think that the cars today weigh less when in fact they weigh more due to the luxury things we like to put in the cars and for safety purposes as well. Let's also remember that the S2000 is a RWD car which also adds on parts to the car that are not found on an Integra.