Going back to the original question, I think there are a few factors that seem to make the difference:
1 - reputation of past events. I'm always interested when another riders sings the praises of an event I haven't tried. That's how I ended up making the Cotswold Corker a must-do on my calendar, despite living in London, and why I'm going to try the BCM this year, despite every report I've read suggesting that it is hell on wheels
. Similarly, certain organisers have personal reputations, both positive and negative.
2 - As someone else has mentioned, if the organiser has links with an active local club then the turn out seems to go up.
3 - A location and start time that fits well with railway timetables. There's one event I love doing but I wish it would start just 15 minutes later so that I could take the first train out of London instead of driving to the start. With events I don't already know that might put me off and I suspect may discourage others too.
4 - Weather on the day.
5 - Proximity to other similar events. Some areas have more events than others and so the turnout per event drops. If you live somewhere that holds only one 600 a year you'd better get your entry in early in a PBP year.
