I found a couple of previous threads that dealt with this topic to some degree.

This thread speaks specifically about America's lack of Imperialism in general, but provides some insights as to why they would not want to engage in the Scramble for Africa: http://www.reddit.com/r/AskHistorians/comments/zj0w8/why_did_the_united_states_never_jump_on_the/

This second thread asks directly about the Scramble for Africa, and has a few responses: http://www.reddit.com/r/AskHistorians/comments/1lj7do/why_didnt_the_usa_take_part_in_the_scramble_for/

There is definitely room for this discussion to continue, but I figured these might be useful as a starting point

There are better answers here answering your question fully, but it's important to mention that Liberia was a de facto American colony. The American Colonization Society felt that African slaves should be returned to Africa--as an alternative to abolition--so they sent a small amount of former slaves to the Pepper Coast. These former slaves formed the state of Liberia.