From what little we saw of Carnes last year, he's not even close to Martinez. He looked horrible in the option game, both on his pitches and his reads (not to mention he's much slower than Martinez). Our offense runs a lot of option, so Carnes' inability to run it effectively (or at least close to Martinez' ability) hinders him greatly.
Not to mention Martinez has been the starter for two years now, and has two years left. He has been building a rapport with our current receivers for two years now. That chemistry (especially next year when they've all played three years together) will pay dividends.