The problem with developing content quickly is that it drops the quality of the content.

I'm sure that, if Square-Enix really wanted to, they could spend all of their time developing JUST new gear and that's it. "February is the month of Hats! March will be the month of Shoes!"

But that'd be silly. Also, there'd have to be content for at least 75-80% of the players to enjoy. That goes for Raiders (PvE), Pitbulls (PvP), Lorehounds (Story-focused / RP), Sweatshoppers (Crafting), Cherrypickers (Gathering), and Gold Diggers (Rare Item Collectors). Creating monthly content for one of those groups is feasible, but not all of them, and even then, the dev team has openly alluded to the fact that they're picking and choosing which groups they appeal to with a given patch, though usually largely appealing to the Raiders and the Lorehounds.