It’s been a minute, but off the top of my head:

• The story was ass.
• Not enough content for a “live service” game.
• Enemy levels scaled heavily with your gear level, so better gear didn’t make you feel any more powerful.
• The stats system for the gear was trash. Like, equipping one piece of high level gear and then keeping your starting gear for everything else was legitimately better than trying to get BiS for all of your gear.
• Gear had randomly generated buffs, and those might buff other gear. But sometimes it would you would get a drop for something your class could equip (not every class could use every weapon, for example) but with buffs to a weapon your class couldn’t use.
• For a game that was essentially “be Iron Man” they included an overheat mechanic so you couldn’t just fly wherever you wanted.