Treyarch come primarily from an era where they could make a game, do minimal patches and then produce basic DLC map packs and then their job was done on the whole product. The climate in gaming has changed significantly now with huge daily demands from the community, ever-changing genre trends and more elements in-game to upkeep. Gone were the days they could make a game and the community would be happy for the whole year and the devs wouldn't need to worry about nerfing, balancing, general updates, new regular content, social media engagement, microtransactions etc. MW2 for example managed to cope when the old IW left the studio and aside from the rock glitch, the game maintained itself perfectly. You can't do that these days and if games don't remain fresh, they get abandoned especially on tiresome brands or saturated genres like this.
If the rumours are to be believed too; Treyarch fucked the single player campaign worse than BO3's so Activision pulled the plug and then commissioned an 11th hour battle royale on top of a multiplayer that was supposedly reworked well over a year into full development. This would also give Treyarch an uphill battle on what is already a huge demand to keep everyone happy and their game relevant. The development hell rumours definitely seem true.