The works are discredited by their own errors. As for "shunning", the scientific community is under NO obligation to take anybody seriously. The attention of scientists is something that must be earned. Mitra, far from earning it, has actively destroyed any chance at such attention, because of his trivially erroneous claims.
Besides, if you follow the links AdmSmith provided, you'll see that Mitra has received the attention of some physicists. The results of that attention were not flattering to Mitra, to say the least.
If a maverick historian came along and tried to claim that the Roman Empire never existed, or that the United States of America existed before England did, would academic historians be under any obligation to give attention to his work?
Sorry, but I don't see that analogy as quite apt. It seems that Mitra, as wild and inaccurate as his theorizing may have been, did serve to point out an inaccuracy in then-current "belief" by the scientific community.