I'm not having a go at the particular sessions to which Jack was referring as such but just the general trend.

I know there are musicians and singers who could quite easily make the transition to "faster" or more advanced sessions or, at least, be able to do so much of the time.
Of course, this is dependent on the prevailing repertoire and it can take some time to adjust to this but I get the impression that many are just content to plough their own furrow.

It's possibly a social thing too. Many of the emerging "adult" singers and musicians aren't "pubby" types and might prefer the tried and tested company of their friends and fellow SMG etc students. Also, there is "safety in numbers", to a certain extent.
It's also the case(Fairly or unfairly), that many people regard much of the established session and folk club scene as a little cliquey.

Another reason why "The Canon's Gait session" has stagnated is also due to its popularity and sheer size. Players are less inclined to introduce new tunes in front of so many people and there is always a tendency to fall back on standard tunes when a session is really large.