Question

Why are so many fossil land- dwelling mammals found in the western united states, but so few in the eastern part of the country?

Answer 1

It's because the flow of the stream.
The flow of the stream flow towad the north-western direction, so all parts of the fossils that died million years ago ended up to be formed on the western region.
To this date, the western united states provide the majority set of fossils that ever found in united states.

Answer 2

Answer: it is because of the Post-glacial rebound (also called isostatic rebound or crustal rebound) is the elevation of land masses after the lifting of the immense weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound and isostatic depression are phases of glacial isostasy (glacial isostatic adjustment, glacioisostasy), the deformation of the Earth's crust in response to changes in ice mass distribution. In simple words, the crust that had the fossils move to the western part of the USA. Remember that things preserve more in ice.