Couldn't say for sure but I would speculate it has to do with, you can link directly to a video and it won't show a bunch of links to other videos to watch, com

Couldn't say for sure but I would speculate it has to do with, you can link directly to a video and it won't show a bunch of links to other videos to watch, comments, ads, etc. Whereas if you post your serious acting monologue on Youtube, even unlisted, it's going to have a bunch of distractions and videos from other people that you probably don't want the casting director being pulled into. The fact that Vimeo doesn't have the same level of content match might play into it too, if you want to show off a clip from the episode of Modern Family where you had two lines you don't want ABC to pull it. I'm sure network effect drive it as well, if you're trying to break into an industry and you see that lots of already successful people are using Vimeo, you're going to use it too.