*shrug* Probably because the designers hadn’t thought of that connection yet. Pinsir was made a number of years before Heracross; I’d say they just thought stag beetles were cool, and didn’t decide to focus on insect fighting as a source of inspiration until later, when they did Heracross.
Pinsir’s kind of a sad little Pokémon… Heracross is everything he should have been, and now no-one cares about him. So sad…
Pokémaniacal 