There are three reasons behind this:
PD testing is a fairly new technology compared to the usual Hipot (Megger) test. PD testing has matured over the years and today it has already been proven to be much more powerful indicator for insulation defects compared to offline test (like hipot, Megger etc).
PD testing may be the most powerful indicator for detecting insulation defect but it is also the most sophisticated test to be carried out. The interpretations of the results are extremely challenging and require very well trained personnel to do the analysis. Once results are correctly interpreted, PD testing will give the best detailed diagnosis of the insulation condition of equipment compared to any Hipot. Megger test etc.
As PD testing is sophisticated, the cost of inspection is generally much higher than usual Hipot testing. However, the cost of PD testing is much lower nowadays.