Basically because in their eyes "if my ship can't be canon, than neither can yours" is how they roll, and if that's what happens, then that means that their ship is just as valid as the non confirmed ship in their eyes.
So for an example, a lot of Bakudeku shippers are pissed that they didn't end up canon, but since Hori didn't flat out say that Izuchako is canon, then in their eyes Bakudeku is equally as valid as Izuocha.
It literally just comes down to "If I can't have it, you can't have it either!!!!". It's sad

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Like a lot of Bakudeku fans are just simply happy that Hori didn't flat out confirm Izuocha lol. It's genuinely kinda pathetic.