Why erasing my PIC with PICKit2v2 doesn't work?
A bulk erase is sensitive to the chip power supply voltage.  According to the datasheet:


  When using ICSP, the part must be supplied at 4.5V to 5.5V if a bulk erase will be executed.


It could be that your USB supply, once fed through the programmer, is dropping below 4.5V. While it's enough to program the chip, it's not enough to erase it.

You can try powering the chip separately from an external 5V power supply to see if that then allows you to erase the chip.