Now some of this is based on speculation but not without long research and learned education of the existing circumstances;
You have to understand the political atmosphere during the period, as a measure of political threat/pressure both France and Great Britain had hinted that they might take action against the United States in restoring territories back to Mexico/Spain if the US showed force in Mexico/Texas. Now the US realized that these were only political threats and they took them with a grain of salt, as is detailed in several different resources.
However, if war/conflict did result then there needed to be an established line of supply to the region, this making the St. Louis overland line a foreseeable measure. And it was also because of these threats that the United States couldn't risk using enlisted men in the region.
Aboard one of the ships that transferred the refugees to Galveston Island there was a man named, you guessed it, Thomas Beale. Ironically, during this same period there is a Beale who is also a runner for another influential man involved at Galveston Island and South America, this last Beale even securing the delivery Brazilian hardwood from south America for this man. Now I can't positively identify these two Beale's as being the same man but I can, however, directly connect the influential person as holding investment in both “shipments.” But in getting a better feel for things you have to understand the activities of the Corporation, which was actually a shelled privateering and smuggling operation of grand proportions. You can't just continue to stack looted merchandise up on an island without somebody noticing or without turning that merchandise into vital dollars, just as they did with the slaves they took as prizes, yet another reason the US avoided any direct connections, even though Onis and others knew it.