Walmart has made a science out of studying what sells where. If something takes up too much floor space in proportion to the profit margin, they move it out in favor of stuff that does better. Another factor is that yard goods require the presence of an employee, unlike a lot of other items.
When I was young (ancient history) Sears, Wards and Penneys also sold fabric, and you could order it from their catalogs. There were also more local fabric stores, and they were thriving. Almost every woman could sew, and many did at least a little. Times have changed.