How about the following factors?
•        Folk club audience numbers on all nights are smaller than they used to be.
•        The audiences are older and are much more likely to have been involved in singing/playing themselves even if only in the past.
•        Partly in consequence of the smaller audience size, folk clubs are held in smaller venues (and finding any venue at all is getting harder).
•        Concert tickets for "top" artists sell at £15-20.
•        Many singaround/club regulars are unwilling/unable to pay this amount out on a regular basis – they're past the age when they'll pay regardless.
•        Even if "regulars" still buy tickets there aren't enough of them to keep a concert-style club going indefinitely.
•        Even if club organisers fill their (smaller) venues it can be hard to break even given the booking fees "top" artists need. (I'm guessing the days when even your most well known stars would sleep on a couch or floor are over.)

(Concerts in larger venues and festivals can still work because they are big enough to be cost effective, infrequent enough to be viable "treats", and don't have to rely on just a central core of loyal regulars.)

On the concert v singaround topic; most singaround/session participants aren't in it for their 15 minutes of fame but the pleasure of sharing and doing something with others. I've found that, luckily, those who just want a platform to show off on are likely to stick to open mics and a minority of acoustic clubs where they can get up at the front and then leave when they've done their bit. Singarounds don't suit their style – no time for long introductions about your life journey, no PA to hide behind, one song and then move on and usually stay in your seat in the circle. I think the reason many singaround regulars don't go to concerts is not that they miss their 5 minutes of fame; what they miss is being an active part of a group activity.