If I recall, their single show back in 1998 didn't sell out too far in advance (if at all). They haven't been around since, their fanbase is older and many have perhaps "moved on" or simply don't know what to expect seeing them today, and their genre has been rather lifeless since the late '90's (at least in the US, as far as I know). I think all those are factors in why there was no immediate sell-out. I'd still imagine at least one show sells out, the other sells well, and both shows are great.