Like missed your chance in a sense... She wanted exclusivity but for your own reasons did not at the time... So she got fed up and left. Then as time went on you realized what you lost, and wish you were exclusive?
If I didn't want to be exclusive I wouldn't be at the point in came up. I'm looking for a relationship so for me I'd have broken up by this point. If I was looking for "casual" I would just be sleeping around or very clear about my intentions from the off.