Why do you think William Harrison and others ignored the Native
American’s demands that western expansion stop?

It was too profitable not to expand and to cater to the growing population.

Explanation:

USA's white population grew incessantly during the Westward expansion as it was a state sponsored policy to move to the west and displace the indigenous people from their wild but fertile lands. It was simply too profitable for the US to use the land which to them was till now untapped.

There was also no real concern for the life of the native Americans due to the racism prevalent in white America. None of the administrators was really concerned with conservation of their culture.