Well, the reason why CMH-DCA is a weak route is because the fares are so high! Walk-up round-trip on AWA on this route is $730 -- hardly a low-fare offering. Contrast that with America West's CMH-BWI service which is $182 round-trip (incidentally, matching WN's fare. This difference is reflected in the passenger totals; CMH-BWI draws well over twice as many daily passengers as CMH-DCA/IAD -- with an average fare that's over 40% lower.
The main problems with CMH are threefold. One is that WN is present in the market, which limits the fare levels for HP and other airlines in markets where they compete. Most of the airports which aren't reachable on WN from CMH are (1) large network hubs, (2) congested/slot-limited airports like BOS, LGA, DCA, or (3) service markets which can't really support much service to CMH. America West already offers service to most of the airports which could be put in the second category. And that has meant that most of the new service additions have been to other network hubs -- DFW, ATL, YYZ; they're also serving PHL, ORD, and EWR. I have my doubts about the CMH-MCO service, given that DL Express is back on the route and WN serves it as well.
The second problem is that CMH is a relatively small market -- especially for a connecting hub. Adding to that is the fact that it's surrounded by a half-dozen competing hubs, all in stronger markets and with broader arrays of service (2 at ORD, CVG, CLE, PIT, DTW). The two together make it tough to generate high-margin O&D traffic as well as the connecting flow needed to make a network hub work. Add to that the third problem -- CMH, like IND, is one of the most fragmented markets in the U.S.
While CMH does indeed have enough passenger demand to justify daily flights to SEA, SFO, SAN, PDX, the yields are just terrible -- most are in the range of 7.5-9 cents/mile. Even HP's yields on CMH-LAX are about 7.2 cents/mile (or were as of 3Q01).
To be honest, I don't know what is to be done about Columbus -- the area had good growth in the 1990's, but it seems to me that it will still be difficult to make much money there with the network hub business model.