Ive often wondered it myself and a few things Ive come up with.
1. Most if not all of the great lakes are land locked so its like a big pond full of prolific breeders so they are going to be quite crammed in like in the tanks at times so kind of some natural instincts for the fish there, where as the americans are in flowing rivers with off running lakes and various mouths and paths for the fish to get in so there is more to spread out over.
2. Im not 100% on this but from what I have seen, American cichlids are more susceptible to hole in the head which comes from confined quarters and bad water. And I do have something to reflect on this, my Heros Severus came from a guy who lives near me that keeps all kinds of south american cichlids a lot of my dream fish were in his show tank and shed. Now his show tank had 2 foot long geophagus 9 adult severums (a mix of efasticious, severus, rotkiel, belem and other sub species) along with a good sized group of smaller geos and about 6 adult Guinacaras and a few Krobia in there as well. And this was in a 4x2x2, now some of his fish were stunningly healthy and my severus is proof of this as she lived in that tank all her life till she came to me. And trust me this guy knows his stuff and he said to me "I cram them in like africans" but the two big geos had chronic hole in the head and so did one of the sevs.
So in my eyes it is doable, but if its ethical fishkeeping or not I would say no its not. One of the reasons I imagine people with africans started doing it was because they dont have a fantastic array of tank mates to choose from so a few tried it and it worked and became the standard way of doing it. Where as new world cichlid keepers have more options with tank mates.
The only time I think I would overstock a tank with cichlids would be if I did a large tank with nano fish in as often the tiny streams that apistos, tiny characins, hatchets, cories and dwarf knifefish dwell in are often really really crammed in because predators are too big to fit in the creeks so yeah.
And an other point (thinking out loud) some people that keep fish that dont listen/read often cram in fish in unsuitable tanks I saw a juwel 190 corner tank with a mix of sevs, texas cichlids, geophagus braziliensis and a few other cichlids to boot, this was on ebay and the tank was 5 years old and all these fish were adults and clearly too big for the tank but they all looked healthy and in good shape.
I guess it comes down to the age old saying of "are you a fish keeper or a fish haver"...... its definitely a grey area IMO though.
Wills
Thanks for sharing them 
