Because people have small minds and boring lives. 
January 2nd, 2014 at 06:45am
Speaking as a person who identifies as a very fluid asexual (I would say demisexual, but people tend to discredit demisexuality even more, or don't even know what it is for that matter!), the problem that I tend to find is that people just excuse asexuality as being a "late bloomer", which, in some cases may be perfectly true, but I find more often than not that it isn't. All of the asexual people I know are college-aged or older, past the stereotypical era of "finding yourself", and have had more than enough time to "bloom", and they simply just discovered that they are uncomfortable with things of sexual nature, and that's okay.
Like it's been said before, people who are vehemently against asexuality probably carry over some prejudices from other sexualities and just don't want to accept the fact that human beings are creatures constantly changing, that adapt and adjust as their bodies, hearts, and minds feel they need to.
Think of it this way; people are completely fine with nuns being celibate, but when you take the religious factor out, put a label on it, and it becomes a sexual identifier for someone to become more comfortable with themselves, suddenly they're up in arms.