Maybe to differentiate from sea trout when sea trout were plentiful? Or was it the north Americans and Steelheads?
Having said that, I've got a chart on my office wall picturing 21 trout of North America - and just one is named a brown trout!
There are also 5 salmon, 7 char and 3 whitefish including the grayling.
I'm headed up to Lough Melvin shortly where there are Soneghan, Gillaroo and sea trout among the mix. It has been a long debate about what a pure brown trout is by definition. Sub species or different? Certainly habitat makes big changes to the same species.