From what I understand, Snapchat killed it. They sold to Twitter pretty early for like $30M and Twitter just didn't move quick enough.

Also, a lot of the popular viners wanted to be paid in some way, and they didn't really have a clear way for that to happen, so the creators pulled out and it died.

TikTok just did it better. Creators can easily get paid, videos are not forced to be 6 second max, and they executed better.