I want to ask why my friend rejected another friend for a date
Do you think you asking your friend will change things for your other friend?
No, it's too late for regrets, I can't do anything about it.
How protective of you to want to make things right for the rejected friend, but this is also seen as being a busybody. Mind your own, they have their reasons for rejecting that you might not even know about. You can't force someone to date who they don't want to date, you should just encourage the rejected friend that it's their loss and not his.
It's not about the date.
It can be about anything else, really. But my point is that you can't force any of your friend to change their decisions in the hopes of them getting together, as in the end, you will end up having a reputation of being nosy or worse, losing your friendship with both of them if you intervene with this. If they give it a shot, great, but if not, I hope you should respect that.