Same reason as to why certain players have been on the end of stick on here since forever. Gundogan, Rodri, Walker, Mahrez, Sané, Zinchenko, Mendy, Otamendi, Stones, Sterling, Jesus etc. Because it's easy to. There's a lot of complexity behind our issues and it's easier to point the finger than to dive deep and understand things in ways that are more meaningful long term. The reality is most of the negative outbursts are from people who have these pent up negative emotions and just need somewhere to direct it. It's easier to blame a small thing (like a player) than have a discussion about some larger issue. Obviously some are actually being reasonable with their dissenting views but you can tell those apart by how they present their points.
And also people can be stubborn (eg look at Gundogan or Mendy threads).
Go back to the Sterling threads during his recent dry run and you'll have seen multiple posts essentially begging Real Madrid to put an offer in. People have spent years talking about Mendy being a shit defender despite the fact he's for the most part been a very good defender. Gundogan is always shat on for being worse than Fernandinho and yet last night showed it's clearly nothing to do with the personnel. People have been giving Sané shit for wanting to go and yet those same people probably gave Sané shit because he let a ball go out for a throw in a few times. You have people bashing the media for being anti-City but some of the vile on these forums by City fans makes the media seem friendly in comparison. This and the Mendy thread are filled with people with this weird determination not just to write off a City player, but also to be right about it. As if they want them to fail just to say they told you so.
Negativity prompts reactions more too. That's why post-match threads when we win are quiet.
He's not singled out for abuse, because plenty get abuse.