How does the USA, especially one that is robbed of the West Coast in some way, going to develop the infrastructure it needs to support colonies abroad? You need a navy to get your settlers to the colony and to police it for pirates, you need to have colonies supported to begin with for one. The US tried to take the Philippines with poor public support for that particular aspect of the Spanish-American War, the result was the Philippines were basically viewed as a massive liability by the American public and nobody bothered to make the necessary commitments to defend them.
The US has cultural reasons not to get into the colonialism game too, it's not like they have much sympathy towards Africans themselves but the notion of doing basically the same thing the Europeans are doing in Africa will not sit well with people.