why can't this particular toll road be made an interstate? Many other toll roads are ...
Many were grandfathered in. Tolls cannot be collected on an interstate if federal funds were used to build it.
From the US DOT:
After extensive debate, Congress decided in 1956 to authorize the BPR to incorporate toll facilities in the Interstate System to ensure connectivity without added expense. Section 113(a) of the Federal-Aid Highway Act of 1956 stated:
Upon a finding by the Secretary of Commerce that such action will promote the development of an integrated Interstate System, the Secretary is authorized to approve as part of the Interstate System any toll road, bridge, or tunnel, now or hereafter constructed, which meets the standards adopted for the improvement of projects located on the Interstate System, whenever such toll road, bridge, or tunnel is located on a route heretofore or hereafter designated as a part of the Interstate System: Provided, That no Federal-aid highway funds shall be expended for the construction, reconstruction, or improvement of any such toll road except to the extent hereafter permitted by law: Provided further, That no Federal-aid highway funds shall be expended for the construction, reconstruction, or improvement of any such toll bridge or tunnel except to the extent now or hereafter permitted by law. (source-
http://www.fhwa.dot.gov/infrastructure/tollroad.cfm)