Maybe Mastodon has to be pushed actively at the conference.
The last conference I went to was the Software Engineering Camp (@swec) in Fรผrth, Germany with around 60 people and there were some toots here on Mastodon. But I was asked to provide a @glacier Installation, so we had a social wall there we ate, in the main room and in the entrance.
So people were actively encouraged to use it.
At the @OSCo we had also a glacier social wall and many wrote toots. But both are bar camp format.
Did you have a social wall there, too?