There are still tickets available for Los Angeles and New York... but this was a special tour, and I think Hanson knew early on that there wouldn't be enough time for the tour to be extensive in any way.. and because of that it could only be in major centers so they could make sure of ticket sales. Even the addition of a show in the UK was extra.
While it might be nice to have something like this on a more regular basis, I think we have to be practical.
This was a tour to promote a new Christmas album. But more importantly, when Hanson are on a world tour (as it normally turns out when tour dates run into November and December) they are normally promoting a studio album (not that a Christmas album isn't recorded in a studio but by way of distinction...) -- I think the fact that when shows run that late in the year, the fact that setlists have sometimes included Christmas songs, is the best practical effort at that kind of thing.