Well, for one, warheads travel at over 15,000 miles per hour. Even if you launched ANOTHER nuclear warhead at an incoming one, chances are, the nuke would just drop past the incoming one, it wouldn't have enough time to explode, or do it at the right time. It's just an amazingly difficult thing to do.
Not that there IS any reason to develop a anti-missile defense, I think. The reason that nobody has been nuked in the past 50 years is mostly because nobody has any defense against retaliation. MAD. It's a beautiful concept.
