I have used female condoms before a few times! I think that likely they are unpopular because of their availability and inconvenience, but they really suck to use in practice anyway, so I don't think they'd be the more popular option, even if you could get them more conveniently.
A list of pros and cons about female condoms:
Pros
-As a young woman, I've run into a lot of young men who would rather not use condoms or.. and this is the big one... don't put it on the fucking correct way because they're just so excited and have never been educated on how to put on a condom so it's the most effective. This can be remedied by putting the condom ONTO the male yourself so you know the job's done right, but egos are fragile, and some guys seem to think that it's patronizing or something. With female condoms, if you know how to use them correctly, you know that a condom was used, and that it was used in the correct way.
-They heat up to body temperature inside of you. Ever feel like a latex condom is sort of medical feeling? Sort of like a weird remembrance of the last time you had a pelvic exam and a doctor put a gloved finger up your hoohah? Well, that feeling can sort of be avoided with the female condom, because it warms up inside of you. A note, though, polyurethane male condoms also heat up better to body temperature, so a female condom isn't the only possible remedy.
-The external ring can actually feel pretty good against your clit. If the girl is on top, the rolled up part at the end sort of stimulates the clitoris some in a pretty nice way.
-It is slightly more effective at preventing some viruses like herpes because of the semi-coverage of the female's external genitalia
Cons
-It might not fit well with all body types. Lots of girls can't use menstrual cups, even, and female condoms might not work as well for some bodies. Everyone's different.
-The first time I used one (there have only been a handful of times, really, I think I had a three pack and never bought them again) I described the new sensation as sort of like having a small trashbag hanging out of my vagina. Very different. It does heat up to body temperature, and the external part can be used to increase stimulation, but in general it's not a feeling I liked very much.
-They're not as easy to put on as a male condom. If every time I had sex I used a female condom, I would probably have to excuse myself to the restroom or something to put it in, because it's not so sexy. Whereas putting on a male condom can be a fun group activity during foreplay, the female condom feels very technical to put in, and is more of a task.
All of this being said, I prefer birth control pills as my pregnancy preventative, and polyurethane male condoms for STD prevention. Would I use female condoms again? Probably not, unless I had a partner who had HSV-2, to reduce the risk of them transmitting it to me. They're too expensive and too much hassle.