It could be a number of things; she might believe that she could be the one to change him, because he is otherwise perfect to her, or she may be in love with the sort of chase of trying to pin a guy down. It isn't always as easy as dropping everything and finding somebody. Establishing a real connection with someone is not only rare, but it takes time, so she could also feel unrelenting because she has invested so much time into this person already.
Personally I don't pursue dudes who don't commit, but that's my theory.
Usually because she also doesn't want commitment, or is just desperate for any guy who is doesn't all the time seek relationships. Mainly because he has more then what guys who want commitment, even though that isn't true. It's her mindset that usually causes this way of thinking.
Because she's infatuated with this guy and he is aesthetically pleasing to the eye. She's been blinded by his good looks, put him on a pedestal and misses possible opportunities with other potential partners. . . well I've heard that it happens a lot. Or maybe it's mutual and she isn't ready for anything serious yet.