Dogs don't generalize well, so even though she knows "sit" in the house, as you've seen, she doesn't know it outside the house. Plus, there are many more distractions outside competing for her attention.
How long have you been working on "sit"? Have you worked with her in multiple rooms? With different distractions (e.g., tv, other people or animals in the room)? It's going to take many, many repetitions of "sit" in the house before she really knows it and will be able to do it in different environments.
Where outside are you trying? It's usually helpful to start in a relative quiet, familiar place like your backyard and then move to busier locations.
You may also need to increase the value of the reinforcer. If you've been using regular dog treats in the house, you may want to try chicken or cheese or hot dogs when you move outside.
And when she does sit, increase the rate of reinforcement. You might give her a treat (or other reinforcer) every time she sits the first time you move outside. The next time, treat every other sit. Then slowly move to a variable ratio schedule.