Flying out of DCA, CLE is better connection then EWR and frequently IAH ( espeically to the west coas and nidwest), However, Channa is right. IAH is like the 6th or 7th largest city in the country with massive business interests including oil, which takes business travelers literally around the globe. I don't knw the percentage but would assume at IAH and EWR, probably between 30-40% of each flight has passengers originating IAH and EWR ( intrenational may even be higher in EWR) so CLE is a smaller city and metro area. There are so many choices on the east coast. Afterall, you can fly many international routes out of CLE via DTW and ORD with one stops. And, the east markets are also heavily covered, Look how many places are served by DL, NW, CO, US in that area. Competition is much stiffer comapred to flights out of IAH to say Lubbock, El Paso, etc.

CLE is a good airport ( could use a club in the RJ terminal) and good city but it is unlikely to become the airline megacity you dream of.