It may be because of couple of reasons:
1-) If she is a virgin, she may be afraid of losing control (like feeling too horny after you touch her from "there") and have sex with you. I mean, she has to stop you at a certain point.
2-) She may be waiting for you to get married?
3-) May be she has some ugly experiences. For example, she may have been sexually abused or may be even raped (and haven't told anyone). If the guy only touches her vagina, then it is logical that she would only react to you touching her from there (and would let you do anything else).
4-) She may be menstruating? Some girls don't like you to learn about their cycles so she may be hiding it. Or, may be she doesn't want you to touch her underwear because you may feel the pad (or feel blood?) which would, in her opinion, totally turn you off and repel you.
5-) She may have a vaginal disease (fungus infection, STD, etc.) and may secretly be under treatment for that.
6-) Some girls say that their vagina doesn't smell good. Some even say that it doesn't taste good. She may just be uncomfortable about you touching her from "down there" if she has such concerns.
7-) Her inner labia may not be symmetrical, or may be there's something else about her vagina that makes it unattractive (in her opinion) to the eye or to your hand (like it doesn't feel nice or something). For example, a small lump there, a pimple, etc.
8-) She may just be embarassed of you touching her vagina. Now, you can partially see her breasts in daily life (like not nipples, but a small portion of it), so she may not be embarassed of you seeing her breasts or nipples, but vagina is different. Even the underwear, when seen, is a huge embarassment for some girls. Now think about vagina!
9-) She may be thinking about whether if you would be turned off because of her pubic hair? Like, may be she didn't shave it and fears that you only like it shaved, or vice versa.
*** These are the reasons that I can come up with. To be sure, just ask her. If she can not answer, it is probably (1), (3), (4), (5) or (6).
I hope this helps! =)
"I know I shouldn't force her but"
Whoa, buddy, don't even go there. You can talk it over like adults, you can ask, you can beg, you can cry, you can offer her a vacation in paradise, but if your mind is wandering to force you need to back off and get a reality check. You might just have to accept that you CAN'T make her more comfortable with it and you have to take no for an answer.
just be straight up. ask her why you aren't allowed. relationships are based on the ability to communicate. maybe a old relationship still gets to her and keeps her from moving so fast.
Just ask her why. In order for her to be more comfortable, though, you have to stop pressuring her.
I would also suggest not asking her right when you're trying to do it. If she moves your hand away and you ask her, it's going to seem like you're pressuring her more. At some point when you're NOT trying to reach below the waist (or doing any intimate touching), ask her.
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