I'm a huge music enthusiast, and I frequently dabble in pop music history, specifically looking at top pop hits over history. Since the Billboard Hot 100 debuted in 1958, there has been a rise in the length of time songs stay at #1 on the chart. I looked into it more closely and made a graph detailing this trend: http://imgur.com/OwoBtBQ (Excel wasn't cooperating, but x-axis is time, for whatever reason it kept it by week #, and y-axis is length of time at #1). It's very clear to see the pattern. It seems to have started in 1991 or 1992. In fact, all of the top 30 longest stays have come since 1990. Is this a consequence of music consumption methods changing with technology? A rise in people wanting to hear songs on the radio? What do you all think?
It's been 10 hours and no one has posted an answer, so I'll run the low-content ban-hammer gauntlet and post a one word answer to your question: SoundScan.
Basically, in May 1991, Billboard's SoundScan replaced the old system of manual estimating of album sales and radio plays, with accurate point-of-sale numbers from big-box retailers like Wal-Mart. Artists who were favorites of record store employees and disk jockeys fell off the charts, while middle-America artists (primarily country) saw their chart positions improve.
Here's a 2001 Billboard article about the 10th anniversary of SoundScan