This creates a social class system surrounding Apple products - only those who are privileged enough to be on the Apple ecosystem are allowed to watch Apple events.

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.