One More reason can be that you have declared a profession in the account, but while applying for a preapproved card, you have selected another option.
This will lead to the rejection of a pre-approved credit card. I have contacted HDFC for the same and they mentioned that if there is any difference between the account and card application, then according to RBI guidelines, it will be rejected.
You can raise concerns with them by mail and they will call you to reprocess your application by correcting the details.