Credit cards usually look at your income to obligations ratio, your credit rating, and your income to figure out whether you’re approved or not. They also like to verify these things before committing one way or another. If they offer online applications with immediate responses, they usually have a range of criteria that means an instant approval, a range of criteria that means an instant rejection, and a range of criteria that means a human being has to review them.
Generally, if you are told that they can’t approve you immediately, it means that they can’t verify all of your information automatically (and thus are punting the application to a human being), that they can verify your information but you’re in the range of people who need human review, or that you’ve been rejected but they don’t want to tell you that immediately.
(This last is often used for store credit cards—the hope is that if the site says “We need to verify some information before we can extend you credit,” that you’ll complete the purchase using a card you already have.)