tl;dr Alienating a person can generate interest when negativity arises. Sometimes the means of alienation can be their skin colour. Combining these can create a narrative, which in turn creates stories that people will WANT to read which in turn generates money.
Throughout my working life I've always been required to convince people to do something. Persuade them into seeing your point of view and having them operate to your will. Sometimes that's with colleagues, sometimes that's with clientele, and sometimes that's with customers, all depending on the job I had.
And one big thing you learn in the process is that people like people who are like them. And conversely people are less willing (subconsciously) to like people who aren't like them. And the media knows this.
Now for one, they're always going to have their targets, and they'll stick to them to build momentum to help create the narrative and generate interest and discussion. And another, they're ALWAYS going to focus on elements on what makes these targets different (or specifically, less like the readers) so that the intended audience remove all sense of similarity (or rapport) with the target. This makes it easier to become invested in the negative aspects and buy into them, or more worryingly, be willing to buy into them. For an idea of what I mean think of how we as City fans react differently to negative stories about Pep Guardiola and similarly negative stories about Jose Mourinho. Pep is one of us and we reject any negativity, whereas Mourinho is the enemy and we embrace such negativity.
Sometimes celebrities can be targeted and alienated because they're rich, but only when they display signs that they are flashy. Maybe like Pogba (although it's not just that with him). Previously Ronaldo The Younger would be a target as he consistently showed signs of massive vanity. Sterling isn't flashy or remotely vain though, and has shown constant humility as a professional. There's a reason he's VERY popular with City fans.
Sometimes they can be alienated because they keep doing bad stuff. Think as to when Beckham got a load of shit; after he got sent off at the world cup, after the whole Rebecca Loos affair business, and the recent knighthood email malarkey. Otherwise he's a saint. They did this with Rooney too. Barring the whole laughing gas thing, Raheem is quite well behaved by all accounts.
But sometimes there's just something about them that means you can keep hammering away at them and people will eat it up. Rooney for example gets a lot of stick for things he does right, but he also got a lot most of the rest of the time anyway. You could say that stemmed from disappointing England performances, but he's hardly alone. But they've always marketed him as this dumb and ugly scouser, and that has removed all sense of similarity with him from the general public. Now the amazing part is, they never have to explicitly state he's a dumb and ugly scouser. They know that's what you think of him and they're playing off it effortlessly.
So bringing back to my first point, remember again I said people like people who are like them, and are less trustworthy and willing to like people who aren't. But Sterling really isn't that different, at least as a person. He's well behaved, family orientated, humble, softly spoken and a consummate professional. If anything that makes him likeable. But bearing in mind the stereotypical audiences of the two main anti-Sterling fronts (The Sun and The Mail), there's a rather obvious aspect of him that stands out as different, notably his place of birth and his skin colour. And like Rooney being a dumb and ugly scouser they never have to mention it. But it's there. And we know plenty of people have some innate racist tones, at least in the sense of seeing people other than themselves as different (through no means other than their upbringing and not because they're evil in case you wondered), it came out last night that a higher up City recruitment manager was referring to black players as BBQs. So why wouldn't plenty of other reasonable people see it as a us and them kind of thing with a black person?
I need to be clear, the majority of the public aren't the ones who are being racist. When they say he's a bad footballer it's because that's the narrative they've bought. When Lee Dixon targets Sterling and praises Lingard and Alli despite the fact all three were the same, it's because he's incapable of objective and uninfluenced thought. When Mark Ogden writes a column pining for Rashford, it's exactly the same.
And I agree with plenty of others that the transfer from Liverpool was what absolutely stoked the fire to really magnify the hate (although the whole laughing gas thing was the original kickstart). But the media, who we know many of have little moral in the pursuit of money, are rather cynically using these racist undertones as a means of getting ahead.
Honestly ask yourself, would it be the same if he were white? Because unless the answer is 100% yes than that makes Wrighty right.