It's sadly not that complex.
First, we had a lot of younger starters coming into this year. However, despite that fact Sark decided to run "NFL style" spring and summer drills which are essentially non-contact. The thought behind this is that it reduced chances for injury. It can work in the NFL because there is almost no season where a team places 3-5 starters on the OL. It doesn't work in college because OL is among the latest developing positions in college football. It also has some of the largest misses because the physical changes between 18-22 of men that large can great decrease or enhance their ability to execute the game plan. Second, because you don't get to see them in full speed contact drills it becomes nearly impossible to make the proper evaluations of growth and development among those type of players. As an important side note, the exact same is true of the QB position regarding growth and evaluation in college. If a QB isn't seeing real world speeds and pressures you can't evaluate his real world growth and ability.
That huge piece said, there is also a critical aspect of aligning your game plan to the skillset and abilities of your players. Because you can have wholesale changes at QB and skill positions in a single year and need to replace 3-5 starters on OL, what worked with one group of OL, one group of skill players and one QB may not work well or at all with an entirely different set of each. Texas faced all these save RB who we returned. If you haven't seen them in full speed practices in summer and fall camps it's absolutely impossible to make these adjustments because you can't evaluate the skills and abilities of the players.
So, you have to wait for actual games to see both these faults, weaknesses and strengths. Ohio State made them all very clear. However, despite that reality and given we then faced 3 teams we absolutely outmatched those adjustments were not nessecary in order to win convincingly. So, basically no changes were made to blocking scheme, passing or running game.
Which leads us to Florida. Sark is a very easy read for a competent DC and HC. His habits and play calls are extremely predictable. While Florida is a struggling team they do have talent physically on the defense. So, a coach who is about to get fired schemed against a predictable offensive game plan that our players couldn't execute very well. The result were exactly what you saw.
WHICH lead Sark to alter blocking schemes, QB play, RB play for OU. He simplified blocking from complex strategies and long developing WR routes to shorter routes, very non-complex down blocking schemes, shorter much quicker WR routes, alternative running lanes that were quick hitting and didn't make his young QB turn his back to the field.
It worked really well against a very good OU defense and Texas was greatly helped by an OU offense that could not do jack shit. So, in addition to a less complex scheme that our underdeveloped OL and young QB could better execute, fatigue on OU defense also helped.
Then came Kentucky where Sark went back to the complex blocking, longer WR routes, same predictable RB runs that also develop slowly and a coach who's not a complete idiot prepared for just that. Sark did it because just like the 3 crappy teams we played and incorrectly Florida he thought the talent would over come poor execute by the Texas players against a far less talented team physically in Kentucky.
It didn't. Sark will do the same against the next mid-tier opponent and then just shit the bed against quality ones.
It's what he does and it won't change.
Why did go away from what worked for OU? Cause he's a offensive genius of course. Ego comes before the fall.
Why do we look so bad?
Bad talent evaluation in recruiting, inability to evaluate the talent in spring and practice given NFL style approach, complex blocking schemes and long developing WR routes without the talent and abilities to execute them.
Edit- 3 important additional aspects. Sark does not create offensive game plans to align with the skillet of the player he has at his disposal. This is complicated by a clear inability to evaluate talent (partially due to the NFL practice) and just bad evaluation of talent. Second, Sark does NOT adjust to the defensive scheme of opponents. He doesn't take what the defense gives us, rather he tries to impose his will against defensive schemes, whether it is working or not.