Half marathon as a big event is a fairly recent development. That distance has really exploded in the past 15-20 years. Basically because hobby joggers like to get the finisher medal/hoopla of a full marathon with only half the effort. And they like the word "marathon" in the race title, even when preceded by "half".
Some of the old half marathon records were pretty weak for a long time just because it wasn't run that often and not by the best runners until more recently.