Here's a really good video that talks about the problem you bring up.

Tl;dw the erasure and othering of a culture was an integral part of the original definition of genocide, but when counties were voting on language (I think in the UN) to determine what could be punished as a genocide, many member countries refused to sign a law that included cultural erasure, forced assimilation, and the like because they were afraid that they could be prosecuted under such a law.

So our narrow definition of genocide comes from a process where the criminals got to write the rulebook.