Exactly why she left it off the chalkboard. She knew she couldn't ask for it. Still heartbreaking that for all these years that's all she's asked for in return for saving the Doctor. She wanted him to stay the Doctor, keep running and remember her. He can piece together bits of her, know her name, some of their…

I also loved that ultimately Moffat played with Clara's catch phrase. Over and over, Clara initially repeated "Run you clever boy, and remember me." So the one thing that Clara repeatedly asked for was the one thing she doesn't get. The Doctor forgets her.