What is it that prevents using a 3rd party SSD in a 2015 rMBP?
The reason for the "incompatibility" is simple: there is no SATA-based SSD in this Mac. Apple is transitioning away from SATA-Based SSD drives and moving towards PCIe based SSD drives.

I thought there were a couple of manufacturers that had upgrade parts for PCIe SSDs and a quick search for "MacBook Pro PCIe SSD" should yield some results for you.