A few reasons (most of which have been already said):
1) There aren't many Southamericans here.
2) It takes time and commitment to write a TL, specially one in anonther language. Maverick was great doing so, unfortunately he's no longer here.
3) There are many interesting PODs, the thing is, I don't know if most of you'd find them interesting, since it wouldn't change that much worldwide. For example, last month I was reading a book about the colonization of Northwestern Argentina in the second past of the XVI century, I period I knew almost anything (I knew about the colonization of the eastern part of Argentina, whose first settlers came directly from Spain and I had a vague idea that the Northwest was colonized by settlers comming from Perú, but I didn't knew much more). When I read it, I came across very interesting PODs that could have delayed the colonization of the region for quite a long while, maybe even for a century or more. But then what? Surrounded by Spanish settlers in Eastern Argentina, Bolivia and in Chile, I don't see what could the natives do that would change anything else worldwide.
And that happens to many ideas I come across. There will always be readirers interested in TLs where American history goes slighlty different, but how many would be interesting in reading about a slightly different colonizatrion of the Pampas in the XIX century, or a different outcome of Argentinian civil wars in the first half of the XIX century.
4) I don't know how it is in the rest of South American country, but at least in Argentina, we aren't taught much about the history of our neighbours. We know more about the history of Europe and the US than about the history of Prú, Colombia or Venezuela in the XIXth and XXth centuries. We don't know even that much about brazil or even Chile. So, while we could have an easier access to materials about these countries than most of those who live in the US or Europe, and we can also read it easily, we'd still have to do a lot of research to write a TL about any South American country other than our own (Again, I don't know if this is the same for other Southamerican countries).
5) There's not that much info about Precolumbian peoples (which many would find interesting) and the info that exists is extremely fragmented. You've got the Chronics of the conquerors, you've got some written by modern antropologists, you've got some by archeologists, but the information is too scattered. The info that does exists is very specific, scholarly, and not always available online. For instance, I've always wondered if an authoctonous civilization could have existed in the river plate bassin, but I don't know where to find if there were native plants that could have been domesticated, nor if the land that borders Partaná River is as fertile as the one that borders the Eufrates, for example.