Because, up til his sudden character-development-shift, Kaine wasn't just another Peter. Ben during the Clone Saga was just Peter with a stubble. Peter was made into a mass of complexes and Ben was made to look better by comparison by being...well...normal. He was more Peter than Peter during one time.
Kaine had a cooler costume, an obsessive one-track minded vendetta, and added a villain (at the time) I liked to the rogue gallery. See also Venom.