IIUC, the Super Hornet was in response to needing more range, plus a bigger wing (in order to keep the wing loading low) to carry the increase in fuel, the expected increase in size of the new ordnance coming into service, and an increase in overall ordnance load. Apparently, with the desired weapons loads the F/A-18A/B/C/D was at the limits of what was considered acceptable in terms of TO and stall speeds when operating from carriers, and was unacceptable in range - plus carriage of the new ordnance was stressing the wing structure too much.
At the time they were looking at either either buying the F-14D+ Blk whatever variants for combined Fighter/Attack (the Tomcat had more range on internal fuel with an 8,000 lb bomb load than the Hornet did clean), or building an entirely new aircraft, or adapting the F/A-18C/D.
It turned out that in terms of cost effectiveness the F/A-18C/D was not capable of being upgraded or simply improved to provide what was specified (fitting a new wing and larger tail was looked at but eventually discarded).
The F-14D+ would have been able to accomplish the mission(s), but was a maintenance hog (take a look at the variable geometry wing system and the air inlet system), so a new airframe was chosen.
The Super Hornet is really not an updated Hornet - it is a 95%(?) new airframe with almost no parts in common with the FA-18A/B/C/D Hornet. The overall planform of the Hornet was considered good so the appearance of the Super Hornet is that of a bigger Hornet. The last number I saw said that maintenance is estimated to be about 50% of what the F-14D+ Blk whatever would have been.