This is unlike other "proper" conspiracy theories that claim to know the facts, and present made-up evidence to back up their claims, while ignoring any actual scientific proof that works against them.
Critics can ridicule his claim that an advanced ice-age civilization existed all they want (and there are good counter arguments that do just that[1]), but it's certainly curious that advanced structures were built by what modern history tells us should've been hunter-gatherer groups, millennia before the birth of the oldest civilizations we know about. This is worth thinking about, but more importantly, scientifically researching, so that we can have a better understanding of our past and ancestors.
The more Hancock is ignored, ridiculed and labeled as a crackpot, the more it feels like the scientific community doesn't want to research this for whatever reason, thus proving some of his points.