Why Optimus Prime is so popular?BECAUSE HE'S ONE SEXY MUSCLEMAN!
No, really. The main Optimus desing show him with a broad, manly chest, with solid square boobs and a six-pack belly. He have big and strong arms and solid legs. Okay, proportion of the legs are inverted, with calves bigger than the tighs, but the overall feel of power is the same. That's why Energon Optimus Prime, while not combined, is mocked as a couch potato.
Still, most OP follow the sexy and powerful muscleman template. Even if you never noticed it before, your reptilian brain did.
Woment are seduced, men are inspired.