Actually, I believe it has to do w/the size & space needs of the CTX machines. Some airports could incorporate them into their in-line baggage systems, some couldn't; those are the ones that generally have the machines stuck off in a corner of the terminal & the pax has to lug their bags over. Case in point-some terminals @ DFW have that configuration, but Terminal D doesn't, which is also the newest terminal @ the airport. Clearly the airport could either configure, or re-configure, the baggage system for that terminal to accommodate the CTX machines prior to it opening. (So rule of thumb, if you are checking in @ DFW for an AA flight, always go to Terminal D & avoid having to lug your luggage over to the TSA.)