That's partially due to history: Vimeo was split out of CollegeHumor, and CollegeHumor became Dropout. (Both were part of IAC and were spun out/sold off.)
Dropout originally only used Vimeo for video distribution. They switched to Vimeo for OTT only after running their own with a team of developers and finding it to be an unexpectedly hard task. I think I learned that in the Sam Reich Hank Green Decoder interview.