The I-35 corridor is the favorite choice because there already exists the highway corridor and it would only take some expansion of the right of way to add additional lanes.
True that Houston is the 6th largest port. But Houston is already connected to I-35 via I-10 west to SAn Antonio, about 200 miles, or going to Dallas also connects I-35 about 250 miles to the North. So Houston is still connected to the I-35 corridor and shipping of goods from the port already uses those highways. I realize there is an I-69 corridor being promoted by other cities like Indy and Memphis and Detroit, but little of the infrastructure has been developed. It is a plan on paper.. The key to the I-35 corridor is that it is almost dead center in the USA and the corridor is already developed, although there would be a few changes for the routes to bypass some areas. Since regress from the highway will be almost non-existant except for ports and cross connecting "international" corridors there will be no need to pass through urban areas not designated as ports. The other part of the plan is the I-66 trans continental highway running East and West which would also be an international port to port connector.
It all looks good on paper.