There’s a number of factors, but the primary one is money. Disney has looked to partner with various country’s tourism boards in the past to expand the World Showcase (and did so twice before, adding Norway and Morocco) but have not yet found any country who was/is willing to help Disney pay to have their country added.
Additionally, Disney has to prioritize their expansion plans to maximize their investments over time, and adding an expanded Fantasyland (as they are doing now) will bring in a lot more visitors than adding another country to Epcot.
Also, there were plans to add an Africa pavilion but when Animal Kingdom opened, with its extensive African-themed areas, that idea was scrapped.