What events led to the closing of the west?

Massive influx of  influx of homesteaders, ranchers, and miners swelled

Explanation:

The West frontiers were initially opened in order to attract as many immigrants as possible to occupy the land that exist In United States.

By the end of the 18th century, the influx of  homesteaders, ranchers, and miners far surpassed the amount of land and job opportunities that's available in the Western Frontier.

So, the government decided to close it before the regions became overpopulated and people forced to do crimes because they couldn't find sources of income.