Acquiring banks and other financial institutions consider some lines of business as more risky than others. It's based on their experience with such companies and business models. Most common high risk businesses those of are pharmaceutical, gambling, or erotic nature, but quite often also others, like e.g. software (due to the fact that it's difficult to prove anything in case of any disputes).
Of course this always depends on a specific case and institution, so we recommend simply asking us directly whether or not we can provide your business with payment services.