This post by HRE summarizes many of the reasons - really the lack of prize money, particularly for time bonuses, led to many top athletes not running all out if the win was in hand. In both 1977 and 1978 Rodgers won NYC in "modest" times (2:11 and 2:12) knowing that Fukuoka was right around the corner - in 1977 he won that too. The only time Rodgers ran a pseudo-paced marathon was the 2:08 in Japan against relay teams on a course that was found to be 200m short, plus that random 1 hour race during the summer on the track where he would have set an AR in the half marathon had he simply run one more lap (He did 12.9 miles in an hour). As HRE stated, Greg Meyer has been very frank that he held back at Boston in 1983 despite extremely favorable conditions.
Yes, it was very weird running USATF-NE XC in the early 2000s and seeing Dinsamo there with the "Athletics East" green singlet.