The domain is broad: "slaverystories" not "americanslaverystories". Hence my recommendation since american slavery is such a small part of the vast history of slavery.
Not every discussion of a subject must be comprehensive--you noted in your comment that the history of slavery is vast. Why is this site owner expected to address all the different instances on his/her site?
There are plenty of sites that discuss the broader history of slavery, or specific instances of it. This one fits in with the mosaic of other sites out there.
This, of course, was the unspoken thought behind your suggestion. You just don't think that the subject is important. American chattel slavery is both unbelievably important and unique in the history of the world. To minimize its importance in the establishment and rise of the US, the history of Africa and Europe, or its importance in world culture is willful ignorance.
If anything, you're trying to 'minimize the importance' of the enslavement of people in practically every civilization on earth, for generations before the US was founded and still continuing today.
At the end of the day though, that's what the site owner wants, I just don't understand your vitriolic responses to people questioning it.
Hardly. People have been enslaving each other throughout the entirety of human history. To suggest that America is somehow exceptional in this regard reeks of ethnocentrism. The Arab slave trade is arguably just as important, but somehow I doubt you condemn residents of the Middle East as harshly as you do whites...