The reason, it turns out, is because they want to maintain compatibility wth existing PS4 systems.
The PS4 Pro is a noticeable, if a little inconsequential, bump up over the standard PS4. Sony have increased the speed of the PS4 CPU, enhanced the GPU’s capabilities, and they have increased the memory bandwidth. What they have not done, however, is increase the memory pool itself- as with the original PS4, the PS4 Pro comes with 8GB of GDDR5 RAM.
There’s a reason for that, as it turns out. Speaking to 4Gamer, Masayasu Ito, head of Sony Interactive Entertainment Engineering Division, noted that a change in RAM would either push developers to make games with the increased memory pool in mind, causing compatibility problems with the standard PS4, or, in an effort to maintain compatibility with existing PS4 systems, be ignored- and both are hardly scenarios that Sony want.
So it makes sense why Sony chose to stay with the memory pool that they have- and honestly, it’s not like 8GB of RAM is an issue of any kind. Not many games come even close to requiring that kind of memory. No, in this case, keeping the RAM as is, while also increasing the memory bandwidth on hand, which is what Sony did, sounds like the best thing for Sony to have done.