Whenever I apply for a credit card or credit line on the internet, I am often given the message that I was not approved, and that I would get a letter via snail mail as to why I was denied. But, when I read the letter, it is vaguely explained. If I have a score of 686, why would they say my score was 715? Furthermore, why do they not say, the minimum requirement is such and such, and explain which of these criteria you did not meet?
There is not much you can do.
Partly, the people you interact with don’t know themself exactly, so they couldn’t tell you even if they wanted to. They’ll tell you something, of course, if you ask, but it is not necessarily the correct reason.
Furthermore, they have no more interest in you - you will not be a customer, so why spend time with you?
The reason might be too complicated to even understand for humans - the decision is basically calculated from multivariant statistical predictions, and similar to neural networks, they get good results but not necessarily the reasons for it.