Well given that this inane remark came from Fight the System/Know the Cause, I think the first reason is obvious: he is unaware that the adjective formed from the noun 'incest' is incestuous, not incestual.
That aside, I believe he's unaware of its meaning, unless he really does think all the people he refers to as sceptics are related to each other and having sex with each other.
On the other hand, he does become overly excited in a juvenile adolescent way when sex is mentioned; so it may be that he feels daring posting a naughty word, even if he did get it wrong. My belief is when he grows up and actually has some sexual experiences (other than sitting jackknifed over his flying fist in his parents' basement) he will calm down.
But if the question is why does he feel the need to offer what he presumably feels is a deeply wounding insult to anyone who disagrees with him (sorry, I'll make that 'with whom he disagrees', because he's never actually offered any opinions with which to disagree) well, that's pretty much his reason for being on this board.
He has never, ever actually offered any advice, thoughts or opinions on the subject of alternative medicine; the nearest I've seen him come to doing do is when he offered three words of advice to someone about their advanced cancer: 'Try some herbs'. He appears to have no knowledge of alternative medicine, and no opinions about it other than 'it works'.
The only actual opinion he has ventured was when he repeatedly stated all women who supported vaccination should be raped, and singled out one of the regular sceptics on here as particularly deserving of rape.
And that's about his level, and really is all he does - unpleasant, juvenile sniping and insults.