I think you have to differentiate between fairway bunkers and greenside bunkers.
With respect to fairway bunkers, I think the modern element of fairness is the culprit.
Bunkers with steep faces that present a clear impediment to the advancement of the ball have been softened, and, new bunkers are built without steep, deep vertical faces in the interest of the broad spectrum of golfers who may find themselves in those bunkers.
As to greenside bunkers, Gene Sarazen started it, but, I think Ping's LOB wedge is the modern day culprit, combined with maintainance practices that demand that superintendents groom the bunkers on a daily basis.
Add to that equation the benign design of modern day greens brought about by increased speeds.
No longer does the golfer have to, not only get out of the bunker, but, carry a sharp ridge, spine or tier, and allow for the break and/or roll generated or prevented by those features. Flat greens have made the task exponentially easier.
On PGA Tour course after course, it's hit it to point X and let the ball roll to point Y, or, hit it to point Z and have the ball stop there.
Sprecialty equipment, perfectly groomed bunkers, without fronting impediments and benign greens have all conspired to make what was supposed to be a penalty, a hazard, routine fare for the PGA Tour golfer.