Genes.
People like to blame genes.
Now there's no doubt whatsoever that genes affect weight, but given the fact that genes haven't changed in the past 100 years and obesity rates have, it's tough to suggest that if you've got an unlucky deck you're fated to be fat.
Regardless of your genes, if you're not eating more calories than you burn you're not likely to gain weight.
Remember too 100 years ago obesity was an incredible rarity - the world back then was truly a phenomenal behavioural weight loss program as it was a program that worked not on an individual level, but rather a societal one.
"Genes load the gun, the environment pulls the trigger" - Dr. George Bray