Halo gameplay is such a sensitive topic where map design matters a lot. 343i just doesnt want to have the next update be 5/7 terrible maps monthly but will sacrifice few months for 2/3 generally good maps.
Same for modes. They arent as consistent population as slayer variants and only last until before people want a slayer-only mode again. Overally just want to focus on the core aspect of Halo gameplay.
But if 343i releases forge soon, they can let player creativity rocket through map creation and modes on our own working with their core game. Imo, Halo 5’s forge had the best map creations and modes. If Infinites forge is any better, that will save the game.