It's the most recent series, so it's natural to make comparisons. It's just always going to happen. SM is also entirely different in story structure from XY, and since XY tends to be hailed as "The greatest region ever!" (outside of this site at least, here DP reigns for terrible reasons), criticism (even valid criticism), comes off as very harsh. Heck, I love XY and I still think the praise needs to be toned down. SM gets a lot of "They changed it, so it sucks" 'criticism'. XY showed how formulaic the series had gotten, so SM was a chance at something new and fresh. To put it more simply, the people who think XY is the best thing to happen to Pokémon tend to hate SM because Ash isn't a big bad*** hero who always swoops in to save the day. In essence, they aren't really critics of SM, but haters. They don't give it a fair shot, or just ignore it if it isn't their cup of tea, they have to actively hate on it and desperately try to convince others to hate it so they feel justified in their stance. Because if people give a balanced view of SM, that would mean there are good parts, and most of the people criticizing SM don't want to admit there are good parts (or at least, they won't admit there are good parts that don't involve Ash).