Each bank has their own business model.
All banks want to have a relationship where they are your sole credit bank. They want those transaction fees with merchants.
Discover has long had a business model where it’s an easy credit card to get for a younger person for their first one or two credit cards.
They take more risk, because younger individuals have lower pay and are riskier borrowers, but they’re hoping to be your main credit bank for life.
If you already a) have many credit cards and b) have high credit limits with other issuers you’re both higher risk and lower reward for them.
A credit line that exceeds your income without too many other assets and a rent/mortgage means you have a greater amount of borrowing power. Even if it’s not all with them, they have to take it into account with the risk you pose as a borrower.
At the same time you’re showing them that you’re likely to pay your transactions with other banks too, so it’s lower reward for them.
Capitol One is the same way.
I don’t think a $50k total credit limit is a good goal for you if your income is only $33k. If you’re spending a lot of effort getting all your credit limits increased to their max, this can easily deter new banks from wanting to issue new lines of credit to you. $10k-$15k range is a good place to be, given your income.
I would try too hard to go past $20k if you want to easily get new credit cards, until your income is higher.
So can someone explain Discover's malfunction? Was my credit score too low? What minimum score do they require?
You are approved or denied credit because of your overall credit profile, not your credit score.
It also seems that you're conflating processing networks with issuers. You aren't getting approved for Visa, MC and Amex cards... you're being approved for cards from different issuers that happen to have cards that work on those processing networks. You aren't being denied because of a processing network. You're being denied for profile-related reasons, which often are cited in your denial letter(s).
Discover is focused on new to credit or people with damaged credit. They might reject you if you have established excellent credit since you're not the target audience.
That seems like the most probable reason. But would you please back that up with sources? I thought it would make good business sense to focus on those with established excellent credit as well.
If they keep rejecting you, then you should have received a mail with the reason for the denials.