Another factor--and I don't really know what to make of it--is the trend back to natural grass. I think that in the seventies, artificial turf was seen as a reason why fielding percentages were improving, since it provided a more uniform surface. That sounds strange now, with almost no artificial turf fields left and fielding percentages continuing to rise, but at the time it was probably true, given the state of some of the natural grass fields in those days. Could it be, though, that the ideal surface for fielding is a well-maintained grass field, which most parks have these days? Something else to look at is the age of the players. Seriously. If the average age of starting players is higher than it used to be--I don't know if this is true, but it might be, since better conditioning means that good players tend to have longer careers now--then you should expect fewer errors. Older players, as a rule, have a smaller range but a higher fielding percentage.