These tend to be regional hubs, for example Delta's hubs are ATL CVG JFK SLC DFW. Notice Three hubs in the eastern time zone. SLC is delta's westernmost hub operation. Geographically it makes sense for the people traveling within/between Idaho/Montana/Washington/Oregon/Nevada/Colorado, and since it can have feeders from there it is more economical to have it as a hub then to have redundant service from cvg/atl/jfk to boi, bil, btm etc.
Same situation for AA at SJU.
Continental probably has cleveland as a hub becuase Air traffic in the New York/Newark area is ridiculously high, so all the small, regional feeder is put into cleveland, where they can shuttle their passengers to EWR and on to the world. IAH is a good hub for them.
These are My opinion; I am no official spokesperson for any airline in this forum