Thanks to some of the previous posters for pointing out: equipment, course conditons, Nicklaus's wins (18), 2nds (19) and top 10 finishes (40) in the majors. Here's another: with the prize money exponentially greater now than what is was in his day (not to mention changes in the culture as a whole), I'll guess that more of Nicklaus's contemporaries were guys who played to WIN, and not to play safe for 10th place (most PGA tourneys now that'll get you around $100,000).
Tiger may surpass him in major championships, but until he does (and that's not looking as much a cinch as it did at one time), this man is NUMBER ONE.
As for the two posters who rated this guy a 1-star: this is the downside of the cheap and easy 'democracy' that the internet provides. Nothing like not knowing your a** from a hole in the wall: you still get your painfully uninformed and moronic comments posted.