I did presentations on this to both AAMC and AACOM meetings on this over a decade ago and reached a few conclusions
1) Specialized Health Majors often have specific basic sciences geared to their major that are not acceptable to schools of fulfillment of the prereqs. For example, They may have Chemistry for Allied Health majors instead of General Chemistry or Nursing A&P instead of Bio-Major level A&P.
2) These specialized courses above also may not be rigorous preparation for MCAT as the more general course would be.
3) Most specialized health majors who apply right from college have the hurdle of a perceived weaker "vocational" background as well as a challenge on their motivation and commitment to a profession.
4) Those special health majors who apply after graduation while working full time, have much less time, energy, and resources to devote to postbacc, MCAT prep, application prep, and, often in relationships and with family, apply to a limited geographic area.
Both #2 #4 are still supported by their applicant mean MCAT score of 498.6, the lowest of all majors and the only one below 500. Matriculants in this major have an 8 point increase to 506.7, still the lowest of all majors. That would also indicate that there is some benefit of being a specialized health major as they get accepted with the lowest mean MCAT.