Because the fit is perfect, it's slim, it provides just enough face down protection, and it covers with top and bottom with precision cut holes for the speaker/mic, headphones, and lightning port.
There are few if any cases like it available, certainly not leather, and certainly not for $39.
The discoloration issue for the lighter colors is the only down side. True, Apple could have chosen a more finished leather, which would have resisted discoloration better, but that would likely have put it at the over $50 mark, and it would likely not have sold nearly as well.
Please educate us as to what else makes the case so "low quality" in your esteemed anonymous internet opinion.