No reason to construct big skyscrapers, for one. Actually Tucson has pretty much missed a lot of opportunities to develop its downtown area. The city has been trying to raise funds to implement grand plans for downtown revitalization but it has never really taken off and the dip in the economy hasn't helped (not sure of all the contributing factors, but I have heard grumblings of funds-mismanagement). From what I hear, the decision to build the city's minor league/spring training sports complexes far away from downtown really was a missed opportunity that has resulted in low attendance and teams pulling out of Tucson altogether whereas it could have really spearheaded downtown revitalization. All is not lost, though. A plan to build a "modern streetcar" transit system linking the U of A with downtown recently cleared a major hurdle by being granted federal funding as part of the recent stimulus package. This should be a major boon to downtown development.
That said, there are plenty of reasons beyond steel and concrete to enjoy Tucson.
Grew up in S. Arizona, former Tucson resident.