A few reasons:
1. The existence of God cannot be proven. While I might understand how some people explore the possibility of an existence of a God, I cannot understand how people have an absolute conviction in the existence of something that cannot be proven. That level of conviction is, by definition, non-rational. I find it frustrating to have a conversation with someone who won't acknowledge that their certainty (in something you cannot be certain of) is a fundamental gap between us. It's even more frustrating when that person tries to persuade me to abandon a well-reasoned understanding of the world for something with no evidence, that asks me to change my life, and has no visible benefit.
2. I can look into history and see many things done in the name of Christianity that were abhorrent things, like the Inquisition and the Crusades. I can look at the present day and see things being done in the name of Christianity that are abhorrent things, like the Waco or the potential death penalty for gays in Uganda. I think that it is reasonable to predict more abhorrent things will be done in the name of Christianity in the future and that is enough reason, in my opinion, to dismantle the institution in protection of human rights.
3. The Bible contains contradictions (Thou shalt not kill / God makes the Israealites a massive war force, love thy neighbor / put sinners out of the congregation, no difference between Jew and Gentile, slave and free, male and female / homosexuality is a sin) that indicate to me Christianity is inconsistent and that the present-day interpretation(s) are merely the most current spin on a social construct that has always been bent to fit the society it's in. I believe that the current interpretations are behind the societal curve and, given the power that Christianity has in the western world right now, that it is slowing the progress of social evolution and hurting individual people in the process.
4. I am offended by the idea that Christians have set up a narrow and arbitrary standard of who is saved and who is not saved and have marked me in a not-saved bucket simply because I don't take actions that I see as nonsensical and potentially harmful. They disregard whatever good I do in the world and instead they often abrasively accuse and threaten me. It's a very unpleasant and negative experience.
5. I witness Christians giving money to those who misuse it, giving power to those who wield it poorly, and supporting those who are hypocritical. I see that they often choose leadership poorly and vehemently and almost hysterically defend their leadership even after the leadership has been proven in scandal and a betrayal of their trust. This provides even more evidence to me that Christians are attached en masse to the idea of their religion, are operating on groupthink rather than individual critical reasoning, and that is distasteful to me and supports my original suspicion that they are clinging to the idea of a God without proof and not because of legitimate open-minded exploration and experience.
Oh, and by the way, I am a lifelong Christian, mostly in the Anglican tradition with some touches of evangelical background. I wrote my post from the perspective of a non-Christian because from that viewpoint I absolutely understand why they would dislike, distrust and disbelieve Christianity. The above are only a few examples of why.
Honestly, as a Christian, if you can't look at the history and current state of Christianity and how it interacts with the non-Christian world and FIGURE OUT why people dislike it, you are not doing nearly enough self-evaluation. The unexamined faith, where you refuse to consider it from all sides and contemplate the potential pitfalls and how you answer them, is no real faith at all.