The lingering singles of 1998

I have a question regarding 1998. There were quite a few # 1 singles that just lingered on and on but hit their peak no longer after it hit the chart. The songs that I’m thinking of are Savage Garden - Truly Madly Deeply, Usher - Nice and Slow, Janet Jackson - Together Again, Will Smith - Gettin’ Jiggy With It, and Next Too Close. This was before the methodology change in late 1998. Just wondering what kept these songs on for so long, especially since they were not even bulleted.