I think the breeders have inadvertently selected for lower protein over time while chasing yield potential and disease resistance.  It seems that it used to be more common to get higher protein in older varieties that didn't generally have the yield potential that the newer ones do.   I used to grow an old Agripro variety called Thunderbolt because of the high protein and test weight.  But I had to quit growing it because the rust started taking it out and it just wouldn't yield with the newer varieties like T112, Denali, etc.  The worst protein I ever had from Thunderbolt was 12.5% and had several years 15% plus.  I dare say that none of the current varieties are likely to do that now.  I know I haven't even approached 15% since giving up on Tbolt.