It likely did not sell well enough to merit a new generation.  Only time will tell. Apple isn't the competition -- the market for truly upgradeable desktop-CPU based notebooks  is a small one, largely served by Clevo -- which sells through boutique brands such as OriginPC, Falcon Northwest, Eurocom, AVA, Sager and many others.  The current model is the X170KM-G if you're looking for this type of system.  It does have a few major advantages over Dell's Area 51m systems -- including truly upgradeable, standard GPU cards. If you're looking for a large footprint desktop, upgradeable replacement, Clevo is the last man standing.  
Is Dell now wanting to emulate Apple "thin" laptops? It used to be that other laptops tried to look like Alienware laptops for desktop replacement.
What happened to the Area-51m R Laptop?