If you go out in a bog and look around, most of the plants there are angiosperms. The non-angiosperms are mainly mosses (capable of surviving on atmospheric deposition, not really producing the sorts of complex structures that can be adapted for carnivory like leaves and roots), ferns, and horsetails. "Why no carnivorous ferns?" seems like an interesting question but it's also kinda like "Why no flowering ferns?" Because you need structures (leaves, glandular trichomes, or roots) that can be exapted for a new purpose and flowering plants seem to have the most plasticity.