About ten years ago, I used to make 3D art with one or two male topping futa pieces, which is a fetish of mine. Then I got requests for futa topping male commissions and nothing for the other way around (which was surprising to me at the time). Once I explained to the requesters I'm not interested in that kind of malexfuta action and thus, don't make that kind of art, they understood completely.
Artists will make what they want to make (or get paid to make), just as fetishists will enjoy what they want to enjoy. If there happens to be fewer male topping futa content out there, its not purely due to a lack of popularity or some socio-gender issue. Its just plain statistics. If more people have a fetish for one thing than another, there will be more content than for the other.