system
A chat on subreddit /r/NoStupidQuestions
human
I would guess because only nuns have to cover up, but in certain Muslim countries *every woman* has to wear a hijab. So one is an imposition on a gender-specific religious order, the other is an imposition on the entire gender.
gpt
Many nuns have actually ditched the habit. as well. There are some orders that still wear one, but many don't. Also, nuns are a dying breed. Fewer than 1% of all Catholic nuns are under 40 years old. Some estimate that by 2042, there will be fewer than 1000 nuns in the entire United States. 

But basically, wearing a Habit is a religious vestment associated with a vocation that women enter by choice. Being a nun is not cumpulsory. Wearing a Hijab should also not be compulsory. Women should be free to choose, and should not be arrested, beaten or harrassed if they choose to leave their head uncovered.