Optimus Primes are often overlavished compared to Megatrons - typically because the "hero" toy sells better than the "villain". Look at movie Optimus Primes compared to Megatrons; the OPs are way better made than the Megatrons IMO. (although I think Animated Megatron is much better than Animated OP).

Could be worse... we could have an entire series that was almost entirely dominated by Autobots and devoid of Decepticons. *cough*Zone*cough*Battlestars*cough*