Complex sentence: In the early history of Vancouver, these people were the majority, but in a manner of a few years, they quickly fell to a small minority, both because of more diseases, displacement from land, and rising European and Asian populations.
Split into simpler sentences: In the early history of Vancouver, these people were the majority. In a few years, they quickly fell to a small minority, both because of more diseases, displacement from land, and rising European and Asian populations.