Does anyone know?

Try waiting for her to OFFER the sit, rather than asking her for it. (It may take awhile the first few times, but she will catch on and begin offering it faster) She is just following a command, rather than choosing to do the behavior on her own, which is why it seems like she is not learning it. Following a command is much different than choosing to do something on her own. As soon as she sits, priase and immediately open the door (no treats necessary, the door opening is the reward here). She will begin to learn that butt-on-floor makes the door open.
The bell is also a wonderful idea, with the added advantage of you knowing that she has to go even from another room.
One word of caution, though... some dogs may learn that they can make the door open by ringing the bell even when they just want to go out to play. Suddenly, the bell is ringing 100x a day! You can help prevent this by taking the dog on lead to the potty spot when first training the bell trick. (bell= potty, not play)

Here is another thing you can try. We hung a bell from our door handle. We have 4 dogs and they will ring the bell to go out. We use a large jingle bell like the kind you see at Christmas. It was actually very easy to teach them this concept and we never used treats. Hang the bell low enough for them to hit it with a paw or their nose. Everytime you take the dog out touch the bell to nose and make it ring. Then say "go outside and go potty". It took about 3 weeks for them to get the idea. Now, no matter where we are in the house we can hear the bell. If I ring the bell it seems to stimulate them to need to go as well.

oh p.s... :) you'll want to slowly reduce the treats on occasions or she'll be sitting at the door all the time..

That was what i was going to suggest to you. Have her sit at the door, then say 'lets go potty' and maybe give her a treat(just a small tidbit ; liver treat etc) then take her directly outside and give her time to do business.. and continue saying go potty..once she does her business.. praise her up bigtime. Repetition and patience is the key and praising them. This has worked for me in the past. Dogs do learn what the word POTTY means.. :) Hope this works for ya.