What did we know about Alderaan, except it was Leia’s home planet and they didn’t have any weapons before KABOOM!
Alderaan was at least involved in the plot as it was a main character’s home planet and Luke and Ben were on a mission to go there.
Also it being a peaceful planet with no weapons made the deed seem more abhorrent.
I will say the fact that the Falcon was going to Alderaan and it went kablooey makes that destruction a bit more impactful, but in TFA we know that the Republic’s senate and fleet are destroyed by the Starkiller, which isn’t nothing.