This would be too simplistic of an explanation without putting forward the lineage of all the medal holders or the big names in sports from South Asia.
Personally i don't think genes have a large part to play here and the abysmal performance of subcontinent is mainly due to under-funding and neglect than anything else. Take for example the wrestling, Gama pehalwan was one of the top wrestler once, Hashim Khan and Jan sher Khan practically ruled squash, Pakistan and India collectively have more world cups in hockey, we have Neeraj Chopra now, India and Pakistan have obtained medals in boxing and wrestling in the past. All of these sports require tough mental and physical fitness.
The real problem here is the absence of ecosystem which can prepare future generation of athletes making it easier for South asian athletes to compete with their western counterparts.
It is too depressing to think that South Asia did not win medals as it should have.