This was already explained several times by developer themselves. P2P (that’s how older CI games work) has some big issues, like ping and the fact that they use one client as server which means it just can’t work in CIU. CIU has dedicated server which controlls pretty much everything and does this very good. So if developer wants to add co-op, they need to contruct the whole co-op system from scratch to make it work on dedicated server.
But after this there comes ton of issues too:
• In CI3-5 game scales enemies’ health depending on amount of connected players (don’t remember for sure, but score might be affected too). But let’s take CIU cowards and slobs, for example: they shoot with some chance when they are attacked so more players → more bullets → more problems with enemies. Of course they have locked amount of possible shots but in single player you also have an option to one shot them using plasma or neutron gun which just disappears if coward’s health is scaled.
• Bosses and AI. No need to explain that in CI games they can attack dead player, so one dude can just stay in one corner and other annihilate the boss.
• How to handle disconnects? How to handle mission results? Should mission result show how many players were at the session? How should it indicate that player was or wasn’t in the game if he just join and leave?
You can theorethically continue this infinitely.