Turbulence is on the increase, or not.
Crews are more turbulence aware due to an increasing litigious culture; greater attention to seat belts, etc.
Modern aircraft are more flexible, even with flight guidance Turb modes.
Higher cruise speeds; modern aircraft have higher rough air speeds - a structural requirement not necessarily for passenger comfort.
More flights, greater route variety, greater exposure to existing levels of turbulence.
... a bit more room on the post card.
It's better to believe in global warming than not ... after Pascal.