Having a bad credit score usually means you've not managed credit well in the past (or that you've had very little credit before).
It could be that you have a history of missing payments, for example. And this is likely to mean your chances of getting approved for a credit card are low.
And if you've no credit history at all – perhaps you’ve never borrowed – you could also find it difficult to get approved, as lenders will have no idea whether or not you’ll pay them back. It's like someone you don't know asking to borrow cash.
There are ways to improve your credit history, or build it from scratch. Read more on how to boost your creditworthiness.