Because what you currently have is "Each card is a unique combination of three suits: Shape[Color], Shape, and Letter."
So, you'll continue to run into trouble as long as you have two suits of non-readable shapes (letters and numbers are readable shapes).
And actually, that makes me realize that the problem isn't a graphic design issue, it's a game design issue: what does that shape represent? If you decide how it fits within the theme (it's a geographic region; it's a political alignment; etc.), then the graphic design problem will solve itself.