That's because Clinton's perjury was a deliberate lie to prevent the prosecutor from finding the truth. Here you have the potential for perjury charges to be brought because someone could not remember the precise details of a telephone conversation from a few years before, the subject of which was something else entirely, and which was subsequently corrected in later testimony. There's a big difference there.
Perjury requires a showing of intent to deceive. Not remembering is not enough.