"Imperialist" refers to the will to rule over a vast territory ( the empire).
The problem with territory is that it is limited. So if multiple entities want to rule over vast territories, they will end up competing with each other over any new territories that still aren't dominated by others. Even if those new territories won't be (immediately) incorporated into the empires, each of the empire would want to exercise power or influence over them.
This leads to conflicts, and rivalry, and was a reason for the tension between the European powers at the onset of WW1. WW1 was in a way a possibility for each of the powers to gain advantage by showing that they're stronger to the others. They believed that the winning power could then claim power and influence over new lands.
Under mercantilism, merchants can only buy from the mother country and their trade becomes limited [as they cannot sell nor buy from other countries but only their mother country]. While under capitalism, they can trade internationally and choose what they want to buy or sell without intervention of a government.
