The argonians are native to a pretty small part of tamriel the same as the imga the difference being they've given us argonians in multiple regions despite this. cant see why the imga dont travel. you'd think they'd follow altmers pretty much anywhere.
I wouldn't call Black Marsh a small part of Tamriel. it's bigger than Valenwood, where the Imga originate from. also, if my lore is correct, The Hist who are worshiped and from where the argonians come from (this I'm not sure of) once covered whole Tamriel and there were no men or mer. it was in the war between the Ehlnofey and wandering Ehlnofey that most of the Hist realm was destroyed. So actually the Hist and argonians originate from whole tamriel in the very beginning.
I'm not sure about it though, I remember reading this a while back.