Where you expecting a poolside page-turner?
First of all, what language did you read it in? Part of the ongoing issues around the book is that the translated versions don't capture the nuance and turns of phrase of the original Dutch.
I think the popularity of the diary was Anne, even if you account for however much Otto Frank altered and sanitized the original, was a genuinely precocious writer (not as in genius, as in more expressive and competent than her age would suggest) who was introspective and self-aware in her writings about essentially ordinary young girl things. The Holocaust is basically just a backdrop to this girl writing candidly about very mundane things about her life and her feelings. It's her ignorance, optimism and naivete about outside events that makes it remembered as a sort of Romantic tragedy. It's also probably the most relatable by modern standards - despite being in hiding the Franks and the other families they hid with were well cared for, while many others were in ghettos or concentration camps dealing much more directly with terrible conditions.
One comparison point would be Etty Hillsume, a Dutch adult who diaried her life in Amsterdam between 1941-43. Her writing is much more explicit, and she comes across as eccentric, impulsive and mercurial and she has long passages rambling about God as she experienced a religious awakening while in a relationship with her mental health consultant turned employer. It's arguably much more interesting, if less widely accessible.
Another problem with other recovered Holocaust diaries of the period was they were less comprehensive (and comprehensible - there are translation disputes and legibility issues everywhere) but many were also written with the express intent of documenting the crimes of the Holocaust, which, especially given the much stronger prevalence of religious and national pride at the time, ironically makes them less credible. Some were
sonderkommandos who assisted with the extermination of other Jews and certain versions of their accounts are so colourful they come across like cope fanfiction.
is it true there was like 6 chapters of her masturbating, I've never read it.
There are multiple versions with successive additions of extra material. The first 1947 edition compiled by Otto Frank and the publisher left out the explicit stuff. Then a Critical Edition from 1986 contains analysis and comparisons with the three known versions of the diary - Otto's 'cleaned up' version, 'B', the original diary as edited and reformatted by Anne herself which was intended for publication, and 'A', the original unedited draft of the diary. Because of translation disputes and because one of the supplementary notebooks was lost, none of these versions are fully complete, consistent or comprehensive, but 'A' and 'B'' contain the parts about let's call it anatomy.
"I'd like to ask Peter whether he knows what girls look like down there. I don't think boys are as complicated as girls. You can easily see what boys look like in photographs or pictures of male nudes, but with women it's different. In women, the genitals, or whatever they're called, are hidden between their legs. Peter has probably never seen a girl up close. Honestly, neither have I. Boys are a lot easier. How on Earth would I go about describing a girl's parts? I can tell from what he said that he doesn't know exactly how it all fits together. He was talking about the cervix, but that's on the inside, where you can't see. Everything's pretty well arranged in us women. Until I was eleven or twelve, I didn't realize there was a second set of labia on the inside, since you couldn't see them. What's even funnier is that I thought urine came out of the clitoris. I asked Mother one time what that little bump was, and she said she didn't know. She can really play dumb when she wants to!"
"When you're standing up, all you see from the front is hair. Between your legs there are two soft, cushiony things, also covered with hair, which press together when you're standing, so you can't see what's inside. They separate when you sit down, and they're very red and quite fleshy on the inside. In the upper part, between the outer labia, there's a fold of skin that, on second thought, looks like a kind of blister. That's the clitoris. Then come the inner labia, which are also pressed together in a kind of crease. When they open up, you can see a fleshy little mound, no bigger than the top of my thumb. The upper part has a couple of small holes in it, which is where the urine comes out. The lower part looks as if it were just skin, and yet that's where the vagina is. You can barely find it, because the folds of skin hide the opening. The hole's so small I can hardly imagine how a man could get in there, much less how a baby could come out. It's hard enough trying to get your index finger inside. That's all there is, but it has an important role!
Yours, Anne M. Frank"
This doesn't include the five pages about Anne's parents that Otto kept until shortly before his death that I think were put into the newest editions of the diary, or the pages that it was discovered Anne had kept hidden that include dirty jokes, which I don't think have been published yet.