Romo hasn't delivered in a big time game but he does have a career QB rating of 95 and just signed a six year contract extension last year, so that is the reasoning, or maybe Jerry just doesn't want a guy with a big personality and a huge Texas fan base to compete with himself.
With pro qb's it is very hard to tell in advance for NFL GM's and scouts who will be successful and in what program. He definitely has his work cut out for him in Cleveland and he went pretty far back for someone with his single-handed performances, but he is small, not particularly fast (but extremely maneuverable, a gamer, a guy who lifts his entire team, a winner), and was questionable in his off the field activities (this might have been a problem only for him in college with NCAA regulations, but we'll see). But being drafted in the first round doesn't say much about a quarterback. The # of busts drafted in the top five is very high. Ryan Leaf, Akili Smith, Vince Young (?), and on and on. And conversely, there have been many stars drafted outside the first round. Tom Brady was drafted in the sixth round, I think, and there have been numerous other late rounders who became stars.