It appears to have been the result of a bet:
“Captain Foster was working for Timothy Meaher, a wealthy Mobile shipyard owner and steamboat captain, who had built Clotilda in 1856 for the lumber trade. Meaher was said to have wagered some “Northern gentlemen” from New England, who likely provided the financing for the illegal venture, that he could successfully smuggle slaves into the US despite the 1807 Act Prohibiting Importation of Slaves. “
I have knowledge of a much more serious and much more extensive effort to accomplish the same thing that was attempted around 1810. The Spanish still owned Florida at the time, and the plan was to create a massive slave transhipment facility in Florida, and then simply smuggle them across the border into the US.
I learned of the details of this plan by chasing after information regarding "Natural Born Citizen." The man who had come up with the plan, and very nearly carried it through was "James McClure."
He was being backed by a bunch of British merchants, but thanks to Ambassador John Armstrong, he ended up in French custody for a year or so, so his plan was thwarted.