Because it's based on morals held by only some of the population, and is hugely ineffective and leaves your children uneducated and uninformed about having safe sex. Remember that, whilst many people believe in waiting until marriage, even fewer actually make it -so the majority of the kids who pass through the sex education program need to know about how to have sex and remain physically and emotionally healthy.
They too need to be able to be with a partner without worrying about STDs or pregnancy or anything else - I'm not married but my partner and I do not worry about either of those things, because we are educated, informed individuals who are able to make responsible choices about our sex lives.
Informing them that abstinance is the only 100% fail-safe way to avoid the emotional and physical dangers of sex is one thing, but children need to be taught that sex is a) not something reserved exclusively for one group of the population who are married, b) that some people don't believe it's necessary to wait until they are married to become sexually active and c) that there are other ways of keeping themselves safe if they do choose to be sexually active.
There's a whole world of sexual activity that goes on besides intercourse that carries with it as many dangers. Does promoting abstinance have a definitive line? Where do you stop - hand holding? Kissing? Anything involving parts exclusive to one gender?