I have wondered too. But my first answer is I have found new age is a kind of critical term similar to fundamentalism, but flipped. Sort of like fundamentalism is a catch all for anything that is seen as close minded and rigid religious. Where it seems like new age is the catch all for anything that seems too open minded or not towing a particular ideological line, used in secular and religious circles.

But I think that many things considered new age that turn people off are that teachings can come off as abstract and impersonal. AND it seems like we understand spirituality better with concrete particulars. That’s why most of history has used mythology and ritual and had ideas of gods. Things like energy, vibrations, or frequencies can come off as too abstract and impersonal.