I think it was the rate of change. Previous waves were small by comparison and had been absorbed/integrated after a period, with a degree of gradual change. Nothing is forever, but Eternal September was abrupt. To a degree there was an echo of it (not on USENET, but on forums and other places) with the advent of the smartphone in 2007, and another with the mass take-up of Internet services in 2016 on the Indian subcontinent by a huge population.