If she was ever caught she would have had it taken from her and she would have been unable to complete her studies. She was a very lawful student most of the time and did not want to be expelled.
From the Prisoner of Azkaban Chapter 21, Hermione’s Secret
Professor McGonagall made me swear I wouldn’t tell anyone. She had to
write all sorts of letters to the Ministry of Magic so I could have
one. She had to tell them that I was a model student, and that I’d
never, ever use it for anything except my studies.
The stakes were high if she was caught using for trivial means. She probably wouldn't have wanted to betray Professor McGonagall's trust, as well. McGonagall vouched for her and would have possibly got into trouble if Hermione was caught acting out with it.
Prisoner of Azkaban Chapter 21 of Hermione's Secret, pg. 393
"Now pay attention", said Dumbledore, speaking very low, and very
clearly. "Sirius is locked in Professor Flitwick's office on the
seventh floor. Thirteenth window from the right of the West Tower. If
all goes well, you will be able to save more than one innocent life
tonight. But remember this, both of you: you must not be seen. Miss
Granger, you know the law- you must know what is at stake....
Also, these links.
Hermione Granger received one from Professor McGonagall in 1993, so
that she could attend more classes in her third year than time would
allow. Since McGonagall made her swear to not tell anyone about
it, she did not mention it to Harry or Ron until the end of the
school year, when she and Harry used it to travel back in time and
save Sirius Black and Buckbeak from certain death. Special
permission from the Ministry of Magic had to be sought to allow
Hermione to use one, but her academic record ensured that permission
Hermione put a lot of things toward her school and she did not want to be expelled. Remember this qoute.
Hermione: Now if you two don't mind, I'm going to bed before either of
you come up with another clever idea to get us killed - or worse,
She probably would have broken the rule if she could justify it with a logical rationale, like if someone was about to die(Sirius Black or Buckbeak). Because when the need presented itself, Hermione didn't hesitate using it then. But she couldn't so she just worked harder cutting the "dumb" courses and cutting out play.