It's not whether Houston and Philly are different. It's whether the world in which radio operates is different now than 50 years ago. Maybe has an all news station launched in Houston in the 60s, instead of in Philly, the outcome might be different.
I'm sure David can provide exact information but it's been a good 30 years (at least) since a successful (by any standard) all news station launched.
There have been at least three attempts to establish network services to allow all news to operate cheaply (NBC NIS, AP, CNN). All three and the stations that used them flopped. Early all news outlets failed to have staying power, such as McLendon's stations and WAVA. All now footnotes in the history of all news radio.
All this suggests that without benefit of strong inertia, all news is no longer a viable format. And even long-standing all news stations like KYW are not as successful as they once were as audiences, revenue and influence erode.