Even with no service in the market, CLE-DAL is 61 pax a day both ways, most of whom already fly on WN via connection. They could probably pull enough pax from CLE-DFW (657 a day) to fill a flight, but would it be worth it to them? Or would they rather start CLE-HOU with the possibility of int'l connections and a less aggressive competitor?. I believe CLE-HOU will come first.
HOU does provide access to connections that cannot be done through DAL on Southwest, including international destinations along with destinations in the South Texas region that see nonstop service to HOU on Southwest but no nonstop service to DAL on Southwest. In addition, DFW is also slightly closer to downtown Dallas than IAH is to downtown Houston, and HOU is also further from IAH (where IAH is on the north side of the city of Houston and where HOU is on the southeast side of Houston) than DFW is from DAL (where both airports are located northwest of downtown Dallas, with DAL being closer to downtown within the Dallas city limits and DFW being located midway between Dallas and Fort Worth).
There was significant demand for Cleveland to Houston flights prior to the United-Continental merger with 260,000 passengers a year who travelled between CLE and IAH 10 years ago, and there might have even been enough demand for Southwest to serve HOU nonstop from CLE 10 years ago even though Southwest decided not to start nonstop service to HOU from CLE back then. Southwest could still add nonstop service to HOU from CLE since Southwest now has international service from HOU, since HOU is closer to most of the major business and tourist destinations in Greater Houston than IAH is, and since HOU still doesn't have any nonstop service from CLE.
DAL is not as far south as HOU is for connections between CLE and Southern California, but Southwest could also provide easier access to Southern California by adding nonstop service from CLE to LAX and SAN and extra nonstops from CLE to LAS, PHX, and DEN. It appears that there might be enough demand for Southwest to do add at least a 2nd nonstop to LAS from CLE and even possibly a 3rd nonstop to LAS from CLE since there are 180,000 passengers a year who travel between CLE and LAS, and since there are currently only 3 daily nonstops to LAS from CLE (1 on Southwest, 1 on Frontier, and 1 in Spirit). Southwest would also be able to provide easier connections to and from California from CLE if it added additional nonstop flights to LAS from CLE.