Print-on-demand, by its very nature, is geared towards small print runs. Hence, profits are very, very small (if at all). We may not have the heavy upfront printing costs, but we’d still need to make back our plant costs (translation/editing/lettering), and small print runs make this difficult to do.
Additionally, physical bookstores are one of our more important markets— and print-on-demand titles would not appear in bookstores (nor in other popular online retailers, depending on the print-on-demand service). Making a niche title even harder to find doesn’t exactly spell a recipe for success… ^_^;