The A300 and the 1301 look very similar, but they have very different dimensions and fitting issues underneath the forend.
The A300UP was designed with input from the Haughts who campaigned for MLOK slots and a more aggressive texture to give folks a forend that could mount a light and a sling out of the box. That required CAD work, prototyping, and most expensively creating the injection molds for the parts. On a per piece basis, injection molded plastic is dirt cheap. But creating the molds that create those cheap pieces most definitely is not. That is the most expensive aspect of doing any polymer gizmo used in firearms.
And I'm betting that the polymer used is done with embedded glass fiber for toughness as well as significant heat-handling capacity, neither of which are cheap and both of which will wear molds much faster than milder plastics. So to get a production run requires creation of multiple molds for multiple machines, and multiple molds per machine because there's only so many parts that a particular mold can make before it is worn to the point that the parts no longer meet spec. With multiple key dimensions that have to be right (like multiple MLOK slots) that means a mold has lots of key areas where wear can junk the mold.
The 300UP was intended to be rolled out with that improvement from the jump. New product, new budget lines, new parts.
The 1301 wasn't rolled out with those improvements...which came from feedback from folks very familiar with the 1301...and because it's different and would require the entire R&D process to create at least an adapter that would allow fitting the UP's forend on it and potentially creating an entirely new set of molds for an entirely new yet similar part in the hopes that people would buy enough to make the venture profitable. That's a big risk.
The short answer is $$$,$$$.