This is decided by the programmers of events in an early stage. One lecture after another, with a panel to answer some questions, that's it. This format is now becoming quickly obsolete, without an alternative model in sight. I would never like to organize such an event. But maybe this is too easy, too simple.
Conferences are about inclusion and exclusion.
Specially the bigger institutions in Europe tend to look first for big names. If they can contribute to the chosen theme, plays a less important role. Visitors will end up with a strange mix of speakers and a conference without much focus. This of course is compensated by the short period of numbness and hallucination when the Cyber Star is on stage and delivers his or her prophetic lecture-performance. This goes together with the general tendency to 'stage' podium discussions and estheticize boring conference halls.