Why was my Capital One credit card application declined?

Every issuer has its own approval standards, so it’s hard to say with any certainty. Nevertheless, your best bet is to apply for a secured credit card. Secured cards are able to essentially guarantee approval regardless of your credit history or income because they require a refundable security deposit of at least $200 that acts as your credit line and prevents you from overspending.

You can compare secured cards here: https://wallethub.com/credit-cards/secured/.

We also recommend checking your credit score and report for free on WalletHub: https://wallethub.com/join/. It’s possible that there’s something in your credit history holding you back, which you don’t know about.

Even if that’s not the case, it’s a good idea to keep track of your credit-building progress.

It's hard to say why your Capital One credit card application got denied.
You should be able to find out, as credit card issuers are required to disclose the reason for your application’s denial in their rejection letter.
If you believe the reasons are unfair, there's no harm in contacting Capital One and asking for reconsideration. You can't do it online, but you can contact them at 800-625-7866.