I'm brazilian and I think I can answer that one. Touring in Brazil these days is doable and profitable but it wasn't that simple back in the day when DS was still active. McCartney and Sinatra did it but they were much more popular than DS over here. Clapton did a gig in Rio in 1990, I was there with my brother, and the infrastructure was precarious compared to what we saw on the OES tour. There was a festival called Hollywood Rock that tried to bring them in 1992 but when the promoters got told the band needed 2 jumbos just for the equipment they quit. I don't blame DS for skipping South America, everything around here was pretty amateur and unprofessional back then.
That being said, things have changed quite a lot and I don't understand why Mark never came back after the STP tour. All concerts were sold out months in advance and our infrastructure is now just as good as anywhere else's. Not counting on festivals here's a list of bands and solo artists who've dropped by over the last few years: Paul McCartney, Eric Clapton, The Police, Rolling Stones, Rod Stewart, Joe Cocker, U2 (argh), BB King, Elton John, Bob Dylan, Roger Waters... John Mayer is arriving in november. Now I ask, why? Why not Brazil, Mark?