For $900 a year you get 7 TB of video and unlimited live streaming. Yes, YouTube is cheaper (free) and brings a large audience. So you have to balance paying for something that's certain or being at the whim of the market maker.
I believe it's because YouTube is as much a content-discovery platform as a content-delivery platform. If you only use Vimeo, very few people will discover your videos.
I don't know why more people don't upload to both, though.
> whim of the market maker
As I understand it, Vimeo are also far from perfect in this regard.
An interesting anecdote: the recent Michael Moore film, Planet of the Humans, was made freely available on YouTube, but was taken down after a copyright complaint. They moved the video to Vimeo, where it remains. (I can't comment on the legitimacy of the copyright complaint, and this is not an endorsement of the film.)
Clips are a neat feature but they're a sharing feature, not a "help me watch" feature.