1. There was a shortage of good new music
Good point, John, but there was actually a shortage of
any new music by the fall of 1988. Michael Jackson, George Michael, Whitney Houston, INXS, Def Leppard, Taylor Dayne, Rick Astley, Richard Marx, and probably a couple more acts that I cannot recall right now were still releasing singles from albums that were all well over a year old by that time. If you were a fan of any of these acts, and had already bought their latest albums, you would have already been familiar with those fourth, fifth, and sixth singles
well before radio started playing them. Since these artists released these singles in rapid-fire succession, thus keeping themselves on the charts almost continuously throughout 1988, listeners who were not fans of these acts simply never got a break from hearing them on the radio. So by reintroducing some old favorites that never really got a chance the first time, radio was giving listeners a break from some of the same artists who were just totally dominating the airwaves in 1988. (I can't really fault the acts for doing this, because most of those singles, even a year after the album release, still became top 10 and #1 hits, but I believe that was largely due to a shortage of competition, except maybe from each other.)