I can fall asleep almost anywhere. I routinely fall asleep in the break room at work. Once, I was helping a friend fix his car, and I fell asleep on his garage floor when he went inside to get water.
But in a hot metal tube tearing through the sky, with my neck all kinked? Get out of here, man.
If it’s hot in the plane, that might be blocking you from sleeping. Personally I’m always cold in planes and fall asleep relatively easily both on planes and elsewhere.
It also helps to be jet-lagged. Knocks you out right quick :P Only applies to return trips though.
Probably why they put the temperature a bit lower than expected, sleeping means people using less oxygen, using toilets less, drinking less, and moving around less. I’m sure the stewards and stewardesses prefer more people to sleep so they have a less hectic workload.