The bypass is a toll road. It would take some doing to get a new toll road designated as an Interstate.
Actually, not a whole lot of doing. Just NCDOT requesting inclusion into the system from FHWA and a route number from AASHTO. Federal law allows new toll roads to be added to the Interstate system, as long as they're built to Interstate standards, connect to an existing Interstate on at least one end, and have a logical termini on the other.