No argument from me as there are always multiple reason for grand events. But this research attempts to discover what were the primary reasons for great numbers of people to move west. Or maybe it's better to say what enabled great numbers of people to move west.
I again recommend.... Read the book.... it is heavily researched and offers the primary reasons in extreme detail.
It was a process, not an event
(Some say the first chapters are pretty intense, but it gets much better as you get into it)