I think it's because it's an entirely worse game than the original after 12 years apart from graphics. Competitive players had their massively used lan play taken away and the intense micromanagement starcraft is famous for toned down and people who just want to play through the campaign have to buy 3 separate ones to cover the costs of cinematics and activision board of directors' gigolo blowjobs.
Alright, I can't comment on the competitive aspect but I really am surprised how many people here play it competitively, but I can say I don't get this bullshit bitching about buying three separate copies. You're not, you're buying SC2 and then two expansions. SC2 contains as many missions as SC1, just not spread out over three races. I guess that rubs people the wrong way, I don't really care that much. I liked Dawn of War's campaign, and that was only like 12 missions and you only played the Space Marines. People are complaining like you only get 10 Terran missions or something. Every RTS ever gets an expansion, and whatever you say about them, Blizzard has NEVER been skimpy with their expansions.