Honestly I'm not sure what those are, but poetry in itself is difficult, free verse is the easiest type/style, speaking from personal experience.
Reason 1: There are more free verse poetry than other types/styles, therefore it has higher chances of getting picked. Reason 2: Any limitations(like "rules" for how it's written) in poetry makes it increasingly more difficult to write at all, so writing good...?
DDs are just a feature, a feature that the community can help control by sending in suggestions. There are four lit CVs you can note with your suggestions (full details including individual suggestion guidelines can be found here: [link]).
Fixed form is difficult because the piece not only has to have interesting content, and good strong writing but also needs to fit into the rules of that form effectively and that the form doesn't dominate the readability. Often you can find with fixed form poems that a lot of pieces feel forced by the format because the writer is focused on that over the content. Getting the right balance is usually where the DD worthy pieces are- but its all about finding them. I believe there are pieces out there, but we need help from you guys to help us find them
Just to echo what's already been said, it's probably due to several factors. 1) The kind of work that tends to get posted; 2) the kind of work that tends to get suggested for DDs; and 3) the work among that pile the gallery directors think is worth featuring. If you want it to change, then you're going to have to suggest poetry or engage in some kind of effort to draw attention to formal poetry on dA. You'd hardly be the first person to make this effort.
I would also not disregard the fact that formal poetry is simply not the current dominant mode in Western literature.
What drove me crazy, for the two and a half years I posted DD's, was the fact that the people who bitched longest and loudest about what I featured were people who NEVER recommended anything to me. If you want to see different DD's, get off your ass, find some work you like, and RECOMMEND them.