For various reasons. First of, because he's popular and Normies like him. So the /co/ntrarians hate him. The Batfags still see him as Batman Lite, and are mad he broke out from comics. Moralfags hate him because he's not "muh goody-two-shoes". Pulpfags hate him because he;s an actual character with flaws and insecurities, and doesn't fit their Ultra-Machsimo Power-Fantasy tastes. Lelfties hate him because he's still a relatively Right-Leaning Centrist Capitalist Patriot, and pretty much "a fucking straight white male".
So, really, Stark is one of those characters that is hated for every single facet of his persona, from different fractions of people. It's an acomplishment, really.