Maybe because DP really did feel like the last hurrah for Ash, with him placing the highest he’d ever been in the League and him being at his strongest at the time. People thought that was a fitting end for Ash, and then the BW companion silhouettes came out, and some people thought he really wouldn’t go to Unova. Because of that, the people who hated him came out of the woodworks for him to get replaced, etc. I remember as well that people were scared of him being replaced too, so it wasn’t just Ash vs. the world at the time.
It seems that way now too, and I remember people being so bitter about the Kalos League, Greninja, and the then new SM Ash that everywhere you went you’d practically see a wanted poster with his name on it. The reason it seems a lot more civil and nice now is because Ash finally won something of note, so that combined with the gratifying nostalgia the win brings has people thinking that maybe he isn’t so bad after all.