One could just as easily ask why we don't see replica Ford Model A's (late 1920s-early 1930s) for sale, given that we could fit them with unleaded fuel-injected V6 engines instead of their original carburated straight-four engines that had 1/3 as much horsepower.
No, it's all about economy.
You have a working design, a hull that's been tried and tested over and over.
Why would you bother to go through years of R&D for a new hull when you already have one waiting to be refitted?
They refitted the Galaxy as a dreadnought yea?
Would you waste time to research new hulls, or would you rather research new weapons with the Borg threat looming?
Looking at it historically, from Constitution to Sovereign ships got a lot less taller and a lot longer.
NX has the same basic design that looks more like a Sovereign than a Galaxy, it's not very tall, not very wide, but definitely long.
I agree this upside down Akira was a design decision to "make the ship look cool", but hey, it's canon now.
think of it this way, with smaller more effective tech available, the addition of replicators (negating food storage) and the removal of conventional showers (and water storage) and all round better facilities as well as a more simplistic space frame and general simplicity of her systems she would make an exceptionaly effective teaching aid at the academy for on ship training.
Older ships have often been retrofitted and run as training ships in Starfleet and giving a ship so widely known as a project for engineering students while functioning as a light escort for the Tactical students as well as limiting the scientific students and forcing them to use hands on solutions to complicated problems all while on a ship that would be considered uncomfortable by modern standards would be an exceptional introduction to long term existence on a starship.
Now consider the Federation at war, they have ships that literately everyone in the fleet has trained on and knows well enough to practically build from scratch that due to constant maintenance and retrofitting in order to keep up the students training levels can be deployed with a minimal crew with little to no effort and you have a ship that can cover holes in your defence readily available.
Take this to the next level, we know that at least one Defiant class ship was issued for use to Red Squad (the USS Valiant) and it was stated during that episode that a number of old ships were kept updated with modern technology for training at the Academy. Whats to say that the Academy's ships were based on the time tested and simple spaceframe of the NX class.
I honestly cannot think of a better training ship given her size and crew requirements.