This is primarily a hardware limitation. I would hope that optimization will lend the ability to have more. Keep in mind though that the main game is only designed for 8.
This isn't a MMO. This isn't Battlefield. This is not a massive PVP/Raid game like Fortnight or Pubg.
This game is intended to be friendly PvE Co-op with a small group.
To date, they have indicated no inclination towards increasing the player count on consoles. Whether this is because they know they won't be able to get the hardware to do it, or some other arbitrary decision is unclear. Doing the math on the console hardware, and game system requirements however leads to indicate the former. Much more information on this is available here...
[h=3]Sticky:
Dedicated Servers or Increased Player Count on Console or Split Screen MP[/h]