What were the reasons behind the spread of the idea
of manifest destiny

The Religious Influence
All the traveling and expansion were part of the spirit of Manifest Destiny, a belief that it was God's will that Americans spread over the entire continent, and to control and populate the country as they see fit.

Abolition of slavery

Iwould say a, due to the fact these routes display many things that would be useful and provide explanations for how states are known.