A couple of reasons, I think.
First is the simple fact that "theism" is not some single thing. It's a simplistic and superficial label that tells you next to something about someone, much like the color of someone's skin or the genitals they happened to have been born with. Theism is simply acceptance of at least one deity and "deity" can literally be anything. To some people, deity is transcendent and watches over us. To some, deity is the land you walk on and the air you breathe. To others, deity is love, or awe, or creative inspiration. Or it might be all of these things and more if you're a polytheist.
Second is that the implication of being opposed to something is that you want to get rid of it. This means we're talking about ripping out what is a key component of some people's way of life and destroying it. To be blunt, it means we're talking about form of cultural genocide. And considering celebrating the gods - the practice of theism that becomes religion - creates beautiful works of art, strengthens communities, and brings meaning to people's lives, I really can't understand why anyone would want to eliminate that. Yeah, okay, so you get those things without theism? Good for you. But other people like it. Leave them alone to their favorite things. I don't burn your Star Wars DVDs, and you don't burn my Star Trek DVDs, yeah?
If you want to talk about specific abuses of theology or religion, that's fine. But the broad brush is not okay with me.