I have to assume his technical knowledge is sound, as I have none, but Keith Ledgerwood has a compelling theory:

It is my belief that MH370 likely flew in the shadow of SIA68 through India and Afghanistan airspace.  As MH370 was flying “dark” without transponder / ADS-B output, SIA68 would have had no knowledge that MH370 was anywhere around and as it entered Indian airspace, it would have shown up as one single blip on the radar with only the transponder information of SIA68 lighting up ATC and military radar screens.