disclaimer, I am part of the volunteer staff that puts on SCaLE
> Even a city as nice as Ottawa gets a little tiresome after several years in a row
are people attending the conference to see the conference content, or to see the city?
This seems like the least important reason to pick a location, especially for a FOSS conference.
"professional" conferences where the attendees are traveling on expense accounts may need more weight on the tourist aspect of things, but where most people are paying their own way, there are other far more important things.
Moving a conference around spreads the travel costs (i.e. one year you may not have to pay much to travel to it as it's going to happen close to you, another year it may happen further away)
Keeping a conference in one location lets the volunteers build experience rather than having to learn 'running conferences 101' from scratch each year. It also makes it easier for a conference to accumulate equipment that they can re-use.
Of course, you need to have a large enough volunteer population so that you don't become completely dependent on a small number of people and burn them out. This is partly a location issue and partly related to how the particular staff handles newcomers (and this can vary from team to team even within one conference)
It's also really hard for a conference to shrink as economic times get tight, and I think that handling that had more to do with the problems of OLS than the fact that people got tired of one city.