He mentions it in the video. They couldn't rely on Elm, which was too young at the time, for their migration away from Flash. The scale of creating a language was too big for Prezi's immediate needs back then
I went to a JS conference at the time and the CTO of Prezi was talking to me about compiling Haskell to JS; he really wanted Haskell. I told him the performance won’t be good. He wasn’t happy with that answer, but I saw in the end that they went with Elm.