It is too cynical to think that institutions recognized that STEM classes by great professors are a rare and valuable resource, and that by sharing them freely they undermined the value of attending one of these elite institutions? Cost alone cannot be the full explanation as existing resources that were excellent have been actively taken down.
I know that UC Berkley's videos were taken down for failing to conform to accessibility laws so that it kind of a special case, but it wouldn't have been impossible to get them subtitled if the motivation were there.
Of course many of these resources can still be found if you search hard enough, and individual Youtube creators have been getting better and better, but I would really be fascinated to hear why there was such an institutional flourish and decline?