One unique factor about Pittsburgh is its academic presence, in both CS/engineering (Carnegie Mellon) and medical fields (University of Pittsburgh). At least CMU has spawned off a number of research startups, has created a burgeoning robotics community (NREC, etc), and has attracted major tech companies to create offices here: Google, Facebook, Uber, Apple, etc. I'm not sure you could say that about many of the other second-tier cities on your list. Human/intellectual capital is a big part of city momentum.