Likely because this is essentially a low tech alternative theory, so it’s harder to attack using claims of “its just science fiction or supernatural mythology” to discount it.
It really isn’t that outlandish so it skirts the realm of the plausible. He is not making suggestions that a bunch of dudes with antigravity devices flew off to the the four corners of the globe and levitated a bunch of big stones around for some grunting troglodytes. What Hancock calls “advanced” is basically “some groups 12k+ years ago had more advanced stone working and astronomical understanding than other groups” and some folks with that knowledge survived this global catastrophe and disseminated that knowledge.
It’s aggravating mainstream archaeologists because they can’t really prove him wrong (just point to their own self established authority) and frankly any new civilization oriented discoveries that date older than the conventional wisdom just serves to make his theory appear more likely and since he is not as credentialed in the discipline as they are, it erodes their authority.