Federal Abstinence Programs fail because their focus is entirely based on stressing the avoidance of STDs and unplanned pregnancies... but humans will be humans and it is extremely difficult to curb the human sex drive on these premises alone.
Besides, your observation is not as pronounced as you seem to indicate because the applicability of federal abstinence programs to homeschoolers is... ummmm..... quite irrelevant. Don't let that little fact escape the argument.
If for example the Federal Government surveyed 50 million students and found that the success rate for their abstinence programs was 1%. That dismal figure doesn't allow anyone to conclude that abstinence programs fail. The data doesn't say that. It suggests federal abstinence programs are failing. Again, 'federal' is the key word.
If on the other hand, 50% of all home-schoolers managed to remain abstinent that would suggest that the parents were doing something right, something better, that their 'programs' (i.e. approach) was better. The fact that their population pool were 80 times smaller than the public school pool would not negate that argument. You seem to think it does... whatever.
And don't fool yourself into thinking that 'safe-sex' is an entirely risk free alternative to abstinence. If I designed the reliability of the refinery's safety systems, or any other critical quality control measure to match the reliability of a condom I would be fired. Those values are unacceptable... not to mention the fact that condoms have minimal effectiveness against disease - a parameter not even factored into the mix.