Hello, and welcome to the forum!
You ask a very good question, and it's one that I can't really answer...but I am looking forward to what others have to say. I've wondered the same thing myself. The stereo receiver may have better components (better power supply, higher quality electronics), and so costs about the same even though it has fewer features. The 5.1 receiver, however, benefits from much higher volume sales, so it might be just as good and is simply cheaper because they can sell so many of them. I don't know which is true.
My advice? If you can, buy both and compare them to see if you like one over the other.