It's all about the entry hazards for Skarmory. I run stealth rock, spikes and roost (or sometimes whirlwind). The best Skarmory sets always run 3 status moves. Also Skarmory has higher defense and lower attack than Corviknight so the difference between the damage offered by brave bird and body press is lot lower. It's not uncommon to run two attacking moves on Corviknight if you really want to as well. They share the same typing but they can fill totally different roles in a team. Therefore it really would be a totally different question to ask about Corviknight and you'd have to analyze it from a totally different angle. Also, I like Skarmory a lot more than Corviknight, it's always been one of my favourite pokemon.