A large part of it is that a lot of fans will blindly follow whatever the sportswriters say, and sportswriters are fucking morons with no actual moral compass, but they pretend to be geniuses and bastions of morality, so players are demonized and idolized at random.
And whether they are idolized or demonized is often dependent on how well they get along with said sportswriters; Frank Thomas was criticized over and over again by the Chicago media for being honest, whereas Sammy Sosa (whom everyone knew was using PEDs) was given a free pass--until he left the team, that is--because he took pictures with Barney and ran around waving to drunken fans in RF.
Michael Jordan cheated on his numerous times, yet his name was conpicuously absent from a 1998
Sports Illustrated article regarding infidelity among athletes and athletes having children out of wedlock, an article that instead focused its attention on Larry Johnson and Shawn Kemp--the latter of whom has apparently fathered more children than is known by the general public.
--Sran