All of these SGBDs are actually toys being sold for more then they are capable of. So developers there have to try to catch up to PostgreSQL before it becomes (even) easier to use and eat their lunch.
Meanwhile, the issues meriting scarce development and, mainly, review time at PostgreSQL are more interesting than accelerating a few workloads in hardware which is not yet in the servers out there. Things like making PostgreSQL even easier to install, set-up and manage, even more ISO SQL compliant, even more capable, even better than NoSQL at NoSQL loads
Now, if you can show your GPU aware PostgreSQL extension or modification, and show it is generally useful enough to merit review time for the next release why not?