The Roman Empire is the empire it began in
Travel by water was quicker and better for long distance. However boats need to be built and are bulky. Travel by land is stealthier. Both have safety risks. Storms and jagged rock play a risk in water transportation. Land transportation can also be more exhausting, and more suspect to pillaging.
It just be like that sometimes
What does the meaning of life mean to you?
The phenomenon that triangular trade most directly encouraged was slavery. The first part of the triangle was the crossing of the Atlantic Ocean by Europeans wishing to sell their products. With the money obtained the europeans would buy slaves; sometimes they would just trade the products they brought directly for slaves. The same european ships would then sail to America to sell the slaves as plantation labourers. With the money obtained the europeans bought raw products like cotton and took them to Europe for further processing. The complete cycle took like a year.