Not everyone wanted to buy a new core set.
New player just getting into the game could have bought the old core set and then felt cheated when he found out the >>CORE SET<< he bought wasn't actually a starter set for the game.
It hurts sales of the old one. For a while now, new players have constantly been told in my area not to buy the old core set because the damage deck in it will no longer be tournament legal.
Players that have taken a break from the game and perhaps doubled up on the old core will hear about the new mandatory damage deck being in another core set they don't want and sigh and say, 'ugh'.
When someone brings a bunch of people to an event borrowing from their collection, and they have enough damage decks total but not enough of either one there is a definite, "Come on dude" situation.
Oh yeah, and primarily, it adds a little more depth to squad building, like how you get to pick 3 obstacles of your choosing.
Edited by ParaGoomba Slayer