Please explain why some people form a dvt (blood clot) on a flight and others don't?

Many factors: Obesity, injury, birth control pills, recent surgery, advanced age, long flight, compressed calf, history of previous clot - all promote a clot. Shorter flight, moving about, stretching, standing every 2-3 hours, younger age - help prevent it.

Some is luck: Airlines tend to keep the cabin humidity very low (it costs them money to humidify) and if you don't drink to keep hydrated (or drink alcohol) you can get dehydrated. Dehydration and inactivity are big risk factors in normal people for blood clots in the leg. Always keep hydrated and move around on long flights or road trips in cars/buses/trains.