Most acts have a very limited shelf life. Music and tastes change rapidly, and artists get quickly played out. These guys had an excellent (and very long) 8 or 9-year run (notching 2 or 3 of the Top 15 songs in chart history), despite being a fairly boring act. They did it all with good material. When the songs ran out so did their time on the top.
Why hasn't Prince had a hit since the mid-'90s, or Stevie Wonder since the mid-'80s???