As the author of this website mentioned, availability is certainly a factor.
On the Front Range of Colorado, tree companies are always cutting down cottonwood trees, so their availability is a plus. It is very difficult to split without a hydraulic splitter, so that can be a problem for most people.
Once it is cured (which takes much less time in Colorado than more humid climates), it has about the same BTU's as pine, which isn't great, but it is easy to burn and regulate.
The locust is right there with oak as far as BTU's are concerned and do grow on the Front Range, but they seem to stay rather healthy and don't get cut down very often.
Firewood people tend to charge 50% to 100% more for locust than for pine or something like cottonwood or willow.