Just throwing thoughts I've had on the subject off and on over the years:
(1) Coaching: Definitely a candidate. Dating back to Mack Brown, player development hasn't followed excellent recruiting and it's shown in the results.
(2) The Conference: Let's face it, the Big XII is not on par with The SEC, Big Ten, and at times not even the ACC. This is proven on the field as our conference champion (usually Oklahoma) never emerges as the National Champion which consistently comes from these other (3) conferences and often in convincing fashion vs our conference champion.
(3) The Quality of Texas high school talent: Urban Meyer himself commented on this one earlier this year, dismissing the notion based on his Ohio State recruiting and having success with Texas talent. Urban pointed to a lack of player development at programs like Texas. Maybe he's right. But it is suspect when the vast majority of Big XII programs recruits Texas talent and end up with similar results. The gap in Big XII talent and the SEC is noticeably massive in annual NFL draft results by conference especially along the offensive and defensive lines in the early round. If pressed for an answer, I'd speculate the truth on the Texas high school talent debate is somewhere between Urban Meyer's assessment and the counterpoint. Spread football has taken over the Texas high school game and has to this day proved inferior without the injection of physicality you more commonly see in the southeast and midwest.
(4) The Country Club: This phrase was coined during the Mack years and is a way of contrasting the Texas player college football experience with say that of Bear Bryant's Junction Boys. If you've already reached the mountain top in your own mind and are enjoying the fruits and accolades of the summit, are you really motivated to develop and be the best you can be. Hoe much does winning really matter to your average Texas 4-star/5-star athlete vs the passed over 3-stars that end up in programs with less tradition and prestige? Seems like moving on from Mack Brown should have solved this problem, but the problem seemingly still pervades despite stories of Pat Moorer's "Pit" and Herman's "Practice for your breakfast" early on in the Strong and Herman tenures. One way of solving this problem is leadership in each position group-- in other words, when the coaches aren't around, leaders on the team keep it together for them.
(5) The Grass is no longer "Greener": Texas does have a major advantage in resources, but in an environment where coaches can still earn multi-million salaries at programs of lower prestige and more importantly, lower expectations, is there really an incentive to take a job like Texas over a mid-tier power 5 or even non-Power 5 conference job?
I could probably drone on, but I'll stop there for now.