I do not feel qualified to offer a substantial answer to this but I believe it is because of many factors:
1) Radio stations have proven they can deliver customers, more so than in the prior decades;
2) Radio stations have done a better job selling the medium;
3) The competitive thinking back in the 2 minute break time period was that it was best to run 4 to 6 short breaks an hour;
4) Stations realized it was best to run fewer breaks with more spots when it came to retaining listeners. Most run two breaks an hour now.
5) Radio values have increased over the years. Buyers were willing to pay higher multiples, so not only did number of units increase but so did rates;
6) Most stations run about the same number of minutes they did then. Today its 6 minutes twice an hour instead of say, 6 breaks at two minutes;
7) The advertisers still get the results they need.
8) Radio has always had its seasonal ups and downs, so getting while the getting good is usually the attitude when stations don't use a grid system raise and lower rates based on ratings and/or demand. Radio has been forced to be more creative in selling, so while you might hear no spots late at night in the past, now a few extra bucks can get you on with more impressions in less desired times. Every little bit helps.
Today's competitive media choices dictate the value and quantity more than they ever did. Likewise radio listening has changed dramatically. Today there are many listening choices for just over the air radio, not to mention non-on-air sources. People listen differently than in the early 1980s, so what once worked now doesn't. Incredible amounts of research have and continue to show what works and that advice is usually followed.
I recall one major market station that ran about 15 minutes of commercials and promos at :45. Inside the break you heard a ticking clock between every couple of spots saying another 45 minutes of wall to wall music was coming up, or list a popular song coming up or an opportunity to win. The went to #1 although they were never ouside the top 5 anyway. I tought it would backfire. I was really wrong on that one.