I think that it's because many people, whether online or in their real, actual lives, have not heeded the exhortation that Paul gave to Timothy: to watch your life and doctrine closely, and to persevere in them in the hopes of saving self and hearers alike.
It is not enough to believe the right things; one actually has to live the right way. There are people who post about their relationship with God and then turn around and have absolutely no problem calling other women the most base, despicable names. If that's what they're able to do online in the comfort of their own homes, imagine how it is in their actual lives.
Amongst many of the reasons that people do not like Christ - that is, aside from the reasons others have already cited, such as resistance to the truth and the simple fact that if people hated Christ, they will hate Christians - is the sad fact that many Christians make a mockery of His teachings through their lives.
Even as a Christian, I find the vast discordance between the beliefs people espouse and their conduct here and in real life to be puzzling, galling, and downright disheartening. I say this not because I'm perfect; I will be the first to admit that I'm not, and I strive every day to lead a life worthy of the calling I've received. But I also made a commitment to myself that I would strive to comport myself online in a manner that brings glory to God. If the extent of your Christianity ends where man can see you, then how weak is your faith!
This is why I do admire
Shimmie a great deal. Even on the very rare occasion that I might disagree with her, her posts are full of grace and fundamental decency extended to others.