I am lucky in being able to get in to most of these free - but you do still spend money at gigs. . Sid Griffin was interviewed by Bob Harris on GLR a few years ago and he voiced his opinion about how London is the only place where you catch touring American artists - either because their touring schedule only allows them two days in the UK or because there isn't a demand for music out of London. . Most of the acts don't have agents - so the tours are booked directly between artist and promoter. To date they've put on Jolene, Joe Pernice, Tandy and the Bloodshot Review with Waco Brothers, Kelly Hogan, Sally Timms and Chris Mills. Promotion is another aspect.
Promoters can only do so much to make as many people as possible aware that a gig is on.
Some venues advertise heavily, e.g. The Borderline in London, and sure enough shows there recently have been well attended despite tough competition elsewhere. Back up from radio is part of the problem. I feel awkward at a gig that is poorly attended. Help promoters and venues by networking in ways they can't. Instead of going to a gig by yourself - take some friends.