So, on PS4, if you use certain weapons during the Pumpkin/Frost Moon events, Lunar Pillars, and ML fight, the game visually grinds to a halt (though it appears to carry on in the background - often resulting in instant death when it resloves) for up to 30 seconds.
I don't know if this is a result of poorly optimised code or because Terraria pushes the PS4 to the limit when a lot of sprites are on screen. Either way, adding something as big as Calamity would push the PS4 well over the edge, resulting in an even more unreliable play.
Furthermore, DLCs are require to meet certain standards for them to be accepted by Sony. This would mean that DR Studios (I assume they would be responsible) would have to bring the code up to that standard and maintain the code. It's one thing doing so with code the company has written, but an entirely different matter with someone else's code.
It would also mean a fork in Calamity - one developed by the Calamity creators and one by DR Studios.