There are several reasons for why people have denied the holocaust. Some have done it because they're neo-nazis. Others because they simply can't get it into their head that something like that could possibly happen in the wonderfully enlightened 20th century (these are the same people that don't blanch in horror when they hear of massacres in Uganda; they simply can't comprehend such a thing). Still, others do it because the holocaust is a major event in the world of the mainstream about which it is often trendy to gasp at in a collective horror, and hence, in an attempt to show their cool, rebel tendencies, disbelieve it.

As pointed out, in many countries it is illegal. Israel and Germany spring to mind (in the latter, it is also illegal to spread NAZI beliefs and so forth). I do not condone or support this one whit: the freedom of speech protects even the most repulsive and monstrous words there ever were. Making it illegal to speak such repulsive words--such as, say, speech about holocaust denial--is every bit as horrible and dangerous as making it illegal to speak of homosexuality, Judaism, or what-have-you.

Denying the holocaust tends to be a gross thing indeed, in both idea and actuality. It asks questions about the colors of smoke from ovens, what the ashes of bodies were used for, if corpses were used for the manufacture of soap. (my skin creeps) Deniers have also argued that the entire holocaust was a myth perpetuated for the sake of the state of Israel; that the testimony of witnesses (both the poor souls who were victims and that of the exterminators) is worthless because it is anecdotal; that there were not enough Jews in all of Europe to account for the number of deaths.

Surprisingly, sci.skeptic (at least when I was there) was the sight of periodic, ongoing flamewars between a few holocaust deniers and everyone else.

Many people have treated the holocaust as a sacred manner, and they have acted as though the discussion of some of its aspects is a desecration. That is a poor view (although I can understand it entirely when there are those who would have its memory eradicated altogether). The fact is, all events of the past are questionable, and a good many of them are sure to suffer from inaccuracy in our recordings. Revising some of their finer points should not be treated as sacrilege, but revising is a far cry away from denying.

Some (like tftv256) ask why the holocaust is continuously brought up, and he just suggests that sleeping dogs should be let to lie. There are many answers, more than I could even begin to offer. Mine is this: man does not change, and he is chillingly apt to repeat the past. The holocaust is one of those events that should be brought up and retold through the ages as a reminder of what was and what should not be ever again.

For the refutation of holocaust deniers and information on the holocuast, see the Nizkor Project: http://veritas.nizkor.org/

I feel obligated to post the URL to a site that purports that the holocaust didn't happen, though I cannot find a URL (what holocaust denial I've been exposed to has been on television and usenet, and a quick google search was fruitless) . Please /msg me if you have one.