To answer why he's disregarded (in Hancocks view, not my own), I'll refer back to one of his Joe Rogan podcasts (although not verbatim).

Essentially, what he has "discovered" would call for a rewrite of the history books in the fields surrounding these topics - fields where established and prominent scientists/historians already have their books published as the forefront source of information on the topics, which are used in colleges/universities all around the world.

So it's about protecting their own works (ergo, protecting their status and income) more than finding out the accuracy of the historical findings.

Again, this is Hancocks view on why, not so much anyone else's.