Basically. This country loves its iconoclasm.Because Sterling represents a successful, strong willed, independant, rich & talented black person, who isn't conforming.
If Rashford had left Utd & come to City, in a big transfer, he would be getting it.
He would also get it if he was white, to some extent, but if a non white person is in that position, it gives the racists a bandwaggon to jump on, where they can do their thing & get away with it.
You'll note in football, the ones the media looks to take down a peg or two are the once in a generation superstars (superstars by any definition), not simply the good ones. It's why players like Rooney, Beckham, Gazza, and now Sterling got/get so much attention whilst all the other simply brilliant but not quite prodigal players seem to be liked (eg Kane, Rashford, Shearer, Gerrard, Lampard etc). The former players are analogous with the modern day City, the latter with the modern day Liverpool.
And as Nev says, any person with any even slight and subconscious racist tendencies is going to have an opportunity to get them out freely with Sterling, or any other black footballer for that matter.