Because people will come to resent the peer pressure exerted on their thought process. "What? You think Jesus was divine! Ha ha ha! You're a dork!"
It all smacks of Grade Seven.
It's one thing to spontaneously laugh at someone for a dumb belief.
It's quite another to systematically mock people as a strategy to eliminate religion from the world.
People want spirituality. They want to look to the transcendent and the supernatural.
Atheists act like this is some kind of unnatural or irrational aspiration, and that wanting those things are a mark of stupidity or weakness.
The problem with atheists is that while many people are quite happy to mock Christianity, they also happen to believe in astrology, or New Age, or Reincarnation, or Pop Native Spirituality, or any number of non-traditional religious beliefs.
So when they start talking about mocking religious people, they probably don't realize they will either mock a whole lot of people who will come to resent their smugness, or perhaps just create a double standard and mock Christians and not everyone else because they're relatively harmless.
Either way, people will see through it.
The problem is that while atheists want to rid the world of religion, they have yet to come up with a belief system that answers the human heart better than that of Christian love as embodied by the Sermon on the Mount.
Oh yes, they point to the Inquisition and the Crusades, etc, etc, but that's their way of deflecting the true nature of the argument-- the validity of what is proposed by the Sermon on the Mount.
Adopting the Beatitudes makes no sense unless you have some kind of transcendent worldview. You can't be "blessed" in being small, weak, persecuted, pure of heart and so forth, unless there is a God to bless you and give your marginal position some kind of inherent meaning.
When the Beatitudes are practiced, those who truly seek peace and truth-- as opposed to some intellectual posture-- can see the positive effects they have on individuals.
Mocking this stuff is not going to do them a whole lot of good. Yes, it will make some cowardly souls renounce, but in the end, people will come to see it for what it is: a superiority complex. It will rub people the wrong way.