There isn’t necessarily wrong with having an interest in one particular medical specialty. Maybe your interest came about because your favorite aunt died of a brain tumor. Or your grandfather had cardiac problems.
But telling a medical school, including a BS/MD program, that your focus is on being a particular specialty is a bad idea.
Because no matter how well you think you may know that specialty, you really don’t know much. The whole point of the traditional third and fourth year of medical school is to explore all of the different specialties to see which you might be best suited for. Medical students will typically spend 4 to 6 weeks doing nothing but learning about one specialty. To come into a medical school without that experience and declare that there is one and only one specialty for you is naive and shows immaturity.
Instead, medical schools like students to have an open mind on the different options available to them when first starting out. That isn’t to say that you can’t have an area that you think you may prefer. That is fine. But keep an open mind about the medical specialty you will focus on until you have enough information to really make an informed decision.
That is the mature approach.