I could be wrong, but I think Twitter was just about to introduce videos to the platform and saw Vine as a competitor. So they shut it down. I've always thought that was a pretty big mistake.

Twitter as an organization was honestly just incompetent. They also bought and then killed Periscope in late 2020 / early 2021... exactly as the world had started to embrace live video streaming as conferences and concerts started to capitulate and go fully online.