The 1915-D Half Dollar is the second most common date in the series -- only the 1892 is more plentiful. Hundreds of Mint State examples have been certified at the MS63 and MS64 levels. Gems (MS65's) are also plentiful, with scores of examples certified over the past thirty years. Even in MS66, the 1915-D is the second-most plentiful date. Only in MS67 does the 1915-D hit the wall -- not a one of this otherwise common date has earned that lofty grade.
The strike quality of this date may have something to do with the dearth of MS67's. Most examples we've seen are somewhat soft on Liberty's lips and on the upper right shield point and the surrounding feathers.