Diseases and Death
First, a lot of people a part of the journey of the Westward Expansion died. Examples of some diseases that killed people are smallpox, malaria, measles Cholera, Yellow fever, and Gold fever etc. Also multiple of people died from dehydration and starvation. Back then the doctors did not have any decent cures or not that much medicine for these diseases so some people just died.
A Pioneer Traveling in a covered wagon
Secondly there was a lot of ways to get to the west. Here are some of the resources to get to the west. The Oregon Trail, Covered Wagons, Ships, Continental Railroad, etc. It was also a extremely long journey so people on ships probably got seasick and threw up a lot. The ship also had very little space for the passengers to stay in and same with their luggage.
Map of the westward expansin