Nate,
A large part of the basic answer is the fact that the Angus breed has a broader gene pool and much greater numbers. With their CAB program and other marketing they have found ways to effectively utilize and market cattle of many diverse types and traits.
What for me would be important to know is, for those cattle with the highest marbling potential, what impact does that have on other traits like muscling, growth rate and size, feed efficiency, etc. as many of these are antagonistic. So can these Angus cattle with the ability to grade Prime also produce carcasses with acceptable muscling and external fat, while converting at an economically viable rate or are we talking about 1050 lb. steers with 9" REA? One thing that previous data has shown is that our cattle can compete across almost all breeds when put on a constant end point basis (i.e. 1250 lb, 16 months old steers with .5 in fat).
I'm glad you raised the question -- it's an issue we need to look at and something the breed should pay attention to. Good luck in your search.
The other thing to keep in mind, due to numbers, is that Angus breed has far more 2-3 frames (and they had more 8-10 frames when we were all liking them big), has more genetic mutations, more high WW and YW cattle, etc. No matter what trait your looking for (outside color) the Angus breed will have more. That's not an indictment on Shorthorns, it's just a reality.