Does it really still provide great ad revenue? Hasn't that been declining and increasingly restricted over the last several years?
Viewers largely don't care what platform your content is on as long as they can know it exists (link) and can easily consume it without jumping hurdles.
I don't care if I watch something on Youtube or Vimeo as long as I see what I'm trying to see.
YouTube provides easy, great advertising revenue. But as individual channels develop their subscriber base, YouTube's role diminishes.
Viewers/subscribers find it easy to "juggle" multiple apps. People just needed a reason for alternatives.