Why do some people form a dvt (blood clot) on a flight and others don't?

Many reasons: There are many reason for forming blood clots. Prolonged sitting on a plane is a co-factor which may be the "last straw" which causes the clot. Obesity, poor circulation, genetic conditions , lifestyle problems ( smoking) and certain medications are among the many causes of dvt. It is best to discuss your potential risks with your provider before any long plane or car trip.

Many reasons: There are many reasons, as dr. Toppin said in her very nice answer. Much of the reason why people develop dvts on flights has to do with immobility. If you don't move around your chance of a clot becomes greater. Wearing graduated compression stockings helps, as does making sure you keep yourself well hydrated. Water is good for you and it makes you walk around to go to the bathroom.