Or, as I used to think on singers nights in Llandudno for example, that everyone taking a turn would be heard.
Yeah, also a factor. I'm sure everyone knows someone who wouldn't be heard beyond the middle four seats on the first row. :-/ Sometimes they're shy; sometimes they're unsure of their material; sometimes they're old and their voice isn't that strong (even though their song choice, playing and knowledge may make them well worth listening to); sometimes they think the breathy "pop styling" sound is what they should aim for; and more often they just weren't naturally gifted with a perfect voice and haven't found out yet about singing lessons for maximising what they've got.
Re what you were saying about noisy pubs, Jon, a club round our way moved locations a couple of years back. Previously it had been a mix of individuals singing with guitars and session tunes, in a very quiet pub with pretty good acoustics. The new place doesn't have great acoustics, it's fairly busy, and the only space for us was right next to the pool tables, so you've got people around who can't be expected to be quiet and listen (they have as much right to a chat over a game of pool as we do to play music). The club kind of stumbled a bit, but the singers dropped out gradually, and the players carried on and brought their friends. It's now evolved into a rather good tune session instead, because that's what suits the environment best. We could have used a small amp and mics and tried to keep the singing going, but I think it's much better the way it is.