I wish I had some positive experience to draw on to pass on to you.
I've been in this situation a few times and not yet with a positive turnaround. I connected with several girls - we were very close and lost of warm vibes, affection and respect. But they couldn't cross that bridge.
If I wanted to play amateur psychologist I'd suspect it was based on several primary factors.
1> 'common' perceptions of such arrangements (doomed, unacceptable, complicated, or maybe some bogus moral standpoint etc)
2> Insecurity - the mindset that such things always bring some form of 'competition' and they lacked confidence to engage
3> A need for some element of 'control' - especially veto rights etc. Knowing from the beginning that one is only part of a team, or tribe, changes the political aspirations.
There's others, but these seem to be the most common present in some variable proportion.
I think the only hope is to try to stay friends and try to be as transparent as possible. Let her see that it actually IS workable ! Of course in order to do that, you have to do it right and make it workable