The correct answer is they were rich in resources and were thinly settled.
New Mexico, California, and Texas attracted expansionist in the 1890s because these states were rich in resources and were thinly settled.
In those years, the idea that America had a “manifested destiny” was widely believed by many people in the United States. They knew that these states were practically deserted but had many raw resources that could be exploited. So with this manifested destiny, people believed that these states could be part of America and remake the west with the Eastern traditions of the United States and share the culture and traditions that the Northeastern states had. That is why New Mexico, California, and Texas attracted expansionist in the 1890s because these states were rich in resources and were thinly settled.