Consider this....
Being in a community property state, my husband (a stay at home) gets his credit based on my income because the state sees my income as his. When I purchase something, it is not just mine - it is ours. When I bought the house, even though my husband contributed nary a dime - it is half his. If we were to divorce, he would be owed 50% the price of the home. So, I'd have to "buy him out" or sell the home and give him 50% of the proceeds. If my husband owes the IRS for something (kinda hard to do when you don't make anything; but, still...), I would be obligated to pay it...since we file jointly. And, I can be garnished for his debt. Which makes me really glad that he's anti-loans/credit/debt of any sort. I would come very close to committing homicide if I came home one day to find a bill from a new CC.
However, there are tons of states in which communal property is not a law. Let's imagine you lived in such a state (I know, TN is a community property state - but, work with me, here)...If you were in one of those non-community property states, your husband would not be bound to your financial obligations. His money would not considered yours to do with as you will. If he supported you and gave it to you and paid your debts, that'd be because he wanted to - not because he had to. Instead, you would have to jointly apply for the card so he would then be fiscally obligated to any debt you accrued. In this instance - jointly applying - he is saying that he is committed to paying the debt, regardless of whether you are married or not. Since, in that instance, you aren't bound by the common law legislation, this commitment is something that financial institutions seriously prefer.
Since so many states aren't bound by community property laws, and since you could move to such a state at any time, I can't say I blame them for hesitating to give a homemaker their own card without any assurances of repayment.
I'm not trying to cause offense - as I said, my husband is a stay at home, and I value and cherish all he provides me because of it. I don't know what I'd do without him, honestly. However, playing Devil's advocate - it does make sense why they'd hesitate.