It is an attempt to maintain some balance to the program. I know of more than one small school that before this had traditionally put all woodwinds that were not in guard on marching baritones or mellos.
Had a high school friend go to a major university in another state. He said their brass and percussion were great, but most of the woodwinds sounded like middle schoolers. Found out that in that state, most high schools marched no woodwinds at all. They all went to guard, brass, or percussion. Woodwind kids played their instrument half of the year.
For a while in the late 80s the entire Madison, Wisconsin school district eliminated competitive marching band (and some of the Scouts staff worked in the district), "until they got their concert programs back into shape." This was mostly an issue with huge dropoff in woodwind skill level and numbers due to the over emphasis on competitive marching band. Don't know how long they did that, but it did happen.
Does the UIL rubric guarantee these extremes don't happen? No. But it at least gives people a reason to not neglect the woodwind part of the program for half the year.
Why do we let the UIL dictate marching music?
A great show is a great show with or without woodwind exposure or soloists.
The marching shows should be judged on the merits of the show, not because you checked all the boxes on the side of the judging sheet.
We should be able to play music that is best for our band and still be competitive because we played it well and marched it well even if that means we did not expose our woodwinds or have any solos.
While there is a list of music to choose from for concert season, we have the ability to look at the list and find music that is appropriate for our group and play it at contest and be successful.
I do not feel we are given that opportunity on the marching side once you get past the region level.