This is your answer
The indigenous ancestors are so caught up into speaking a very popular language like English and isn't passing on their original language.It only takes one generation to stop passing down a language for them to have descendants that won't know anything of that language.This can happen by choice or by force.
My example:For example my people (Hebrew Israelites) were forced to forget their language (during slavery) and that's how us African Americans now speak English and most of us don't even know what language our ancestors spoke bc they are too busy trying to be accepted by another set of humans.
Explanation