Does anyone know if there is a technical reason they make this Safari-only?
There is a way to get VLC to open the stream, but I haven't had to do it personally.
While they could definitely make all of their events streamable on a wide variety of devices, this tactic makes everyone not in the ecosystem feel as though they've been let down by the technology which they own.
It also ensures that the audience is more Apple-centric, and the first to hear the conference lectures will already be biased in favor of Apple given that they are already Apple users.
Note: before a fervent product enthusiast thinks about hitting that downvote button, I've typed this comment from my very own MBP.