Mack Brown got lucky, in my opinion. Entered into the program with all the momentum in the world - a new conference, an enormous upset to win the inaugural conference championship, etc., and took advantage of it, turning it into a snowball.
His first six seasons were only a hair more impressive than Bo's current resume as a head coach. Then Vince Young happened.
I can't tell if he got lazy over time, or always was, but the prevailing Texas attitude has always been that they are already superior to everyone else. This leads them to think they can get away with taking all the best high school kids (on paper, not necessarily from thorough evaulation) from their state and their state only and they'll magically produce results.