For a very good reason.
Emma Watson is one of the most respected and beloved actresses on the current scene: her fight for equality between cisheteros-xual men and women, for the rights of trans people, the LGTBI collective and to eradicate poverty in the third world, have made her the young actress a benchmark in the world, and even UN Ambassador.
One of the issues that every actor has to face at some point in his career is whether or not to accept going n-de in one of his jobs. And Emma Watson has also been in the same situation. Many actresses who succeeded young (or as children) try to leave the character that brought them fame behind by accepting more dramatic and gritty roles. Kristen Stewart did it with 'On the Road', Miley Cyrus fought to leave her Hannah Montana image behind... but Emma Watson fights for precisely the opposite: she wants to preserve Hermione above all else.
"I'm not going to take off all my clothes or do cocaine in a movie just to leave Hermione behind her. I don't want people to forget me as Hermione. I'm so proud of her."
So don't expect Emma Watson to accept roles where she has to take her clothes off just because. Although she has admitted that, if in the future, she has to strip because she "adds something to the plot and is basic to the development of the character", obviously she will.
