Russell Blackford explains why in this post from which I taken some excerpts and added bold face:
Some ideas do merit marginalisation, and some opponents do lack intellectual legitimacy. That isn’t to say that these ideas and opponents should be censored. There are many reasons why it is best to allow people to speak their minds. But the political freedom to speak your mind does not entail a right to be taken seriously or given deference, or even to be accorded intellectual legitimacy. Indeed, there are plenty of ideas that people should be free to advocate, but which are so clearly foolish or even repugnant that they will, quite rightly, be ignored or treated with derision. Often, ideas that are treated with respect in one generation come to fall in this category in later generations.
For example, a contemporary defence of slavery would fall on deaf ears. Or it might, depending on its context and the way it was expressed, provoke nervous laughter, scorn, repugnance, or even fear. It would not receive a respectful hearing, and anyone who put this idea forward would instantly lose all intellectual credibility (at least in Australia!).
Furthermore, to take less troubling matters, it is unlikely that anyone advocating public policy by telling us that her proposals are in accordance with the will of Aphrodite or Zeus or Odin would receive a respectful hearing in 21st-century Australia. She should be allowed to put her case, can try to persuade us that it is not so outrageous, that it deserves a respectful hearing, and so on. However, the playing field is tilted against her speech, and again quite properly. The onus is on her to explain why not. Prima facie, the will of Zeus is a very poor reason for public policy, and anyone claiming otherwise will, quite properly, be marginalised in serious policy debates.
Of course, we should not censor someone who wants to defend slavery. They should have that freedom. They can plug away with their arguments and try to persuade us to take them seriously. But the playing field is strongly tilted against them, and quite properly so. The onus is on them to explain why it shouldn’t be. Likewise for someone who advocates torture or suicide bombing or female genital mutilation or forced abortions.
But some ideas, though not censored, should be given only a marginal place in our society. In every generation, we continue to debate which those are. I am hopeful that future generations will include not only the examples I’ve given, such as the ideas of reinstituting slavery or punishing homosexuals, but also such examples as the ludicrous idea that the Earth is only 6000 years old (contrary to all conclusions from rational investigation). Likewise for the idea that there is something even “sinful” about (as opposed to grounds for banning) consenting homosexual conduct between sufficiently mature people, or that “sin” attaches to the use of contraceptives or to masturbation. Like the advocacy of slavery, these foolish ideas no longer deserve a level playing field in our society. Let them be freely derided, ridiculed, and driven to the margins. The sooner, the better.