People are fickle, it's that simple.
It's not really that simple. Very few people were inherently against the concept of P4D initially. It's extremely important to not ignore the context P4D was announced in, because that's what brought the ire. If you look back at the NeoGAF Nov. 24, 2013 Persona thread, the P4D announcement thread, or the PersonaGAF IRC chat logs, a lot of people were unhappy with what was going on when P4D entered the fray for a
multitude of reasons (chibi Persona 3DS spin-off, the entire build-up to the live stream, the 72-hour live stream itself, the fact that the finale of the stream occurred very early in the morning for most and that it was filled with trolling...).
The whole thing was crazy, along with people's mental states. After P5 was announced and the stream ended, the overall opinion for P4D
immediately became "oh, alright, cool."
See:
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P4D announcement thread page 1 to post #108.
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Everything after page 5.
Looking back on it, I might have been overestimating how quickly the opinions settled, but it is noticeable, and the confusion/frustration cooled down really quickly. Looking back on the archives, it seems even you were down on P4D right when it was announced.