Both American and United are probably the only two airlines that fly aircraft as large as the 767/ 757 on frequent transcontinental flights. Transcontinental flights have the most fuel onboard of any domestic flight. Any international flight with more fuel on board would have been a lot harder to hijack because of tougher security measures.
It's obvious why the particular airlines, planes, flights and routes were chosen. Whoever planned this out unfourtunately did a very good job.
Nobody expects the Spanish Inquisition !!