During Tiger's reign at number 1, he was winning many tournaments a year. He had such a massive lead, that a few weeks or even months of poor play would not change the rankings. heck, there was a wider spread between 1 and 2, than today between 1 and 10.
Over the past few years, no one has dominated by winning multiple majors or big events, so the changes in ranking occur, pretty quickly. It's funny right now, that if Luke does not finish 8th or higher, then Rory will retain the number 1 ranking.