Besides Nicklaus and Woods, the biggest factor might be that most of today's American PGA Tour pro's have been playing competitive golf since they were in elementary school. There are very few 6, 7 or 8 year-olds who can hold onto a golf club using the Vardon grip. When your hands are very small, an interlocking grip or baseball grip is the best option.
The Vardon was always considered somewhat of an anti-hook grip, reducing the influence of the right hand. It was less "handsy" than the interlocking or baseball grip. Older, bigger players - or those who took up the game later in life, after playing baseball or other sports - were often taught the Vardon grip as the grip of better, stronger players.
Nicklaus certainly changed that perception as he was hardly a "handsy" player and Woods furthered the case. However, other than McIlroy - who also started golf at a very young age - it seems to me that the overlapping grip is still more prevalent among the top Europeans and many of the Korean LPGA stars.