Firstly, because she really was enjoying the parlour. It mixed two of her favourite activities. Experimental magic and cunnilingus. She didn’t think she had done anything worth being asked to leave but she honestly couldn’t think of any other reason Parkinson would have invited her out. Secondly, it was because Parkinson knew everything, and everyone involved. She could possibly have been eyewitness to everything that Hermione had been a part of and whilst she had delighted the exhibitionism and voyeurism at the time, she wasn’t entirely keen on a single person mentioning it to her face. At all. Ever.