I-74 is off because it is a road of compromise. Several states could not decide the route of I-73, so they used I-74 as a fix. In NC, though it runs mostly north-south, it was created as an east-west connector to fix the dispute with VA and to reconnect it to I-73. The same thing happened/happening in WV and SC.
An example of this would be I-40 and I-85 in NC. They both run together in the middle of the state, but technically 85 is the Atlanta/Petersburg highway, (north-south); where I-40 is the Wilmington/Barstow highway (east-west).
I-73 was originally known as the Detroit-Charleston highway, (north-south). I-74 is in fact an east-west interstate in the midwest, but it became a road of compromise, in order to get I-73 completed in the south. NC uses I-74 as an east-west I-73 connector, then it piggybacks I-73 south to US 74 where it will travel east-west again to Wilmington. SC will split the road, going to Myrtle Beach and reconnecting in Charleston.
All of this boils down to money, the feds created I-73 but they did not fund it. Aside from NC, the states are using existing roads to get it done, and the original I-74 is a causality of this.