My stock response is, "Because I like to". Often, this simple statement of personal preference is accepted - after all, nobody feels the need to listen to endless justifications of someone's choice of music or literature or clothing or food.
While the question ostensibly is, "why do you like it", it's really "why should I do it". We're so used to sales pitch and commercials, to being given reasons for acting certain ways, that it has become a way of thinking. Someone is doing something out of the ordinary - what's the sales message, the slogan?
So if they insist, they're actually asking the second question: "give me a reason why I should do it". At that point the only possible response is: "I can't give you a reason why you should meditate. If you find yourself meditating or having the urge to do so, that's your reason right there. Otherwise, there's no point."
Or, more diplomatically, "I can't make decisions on your behalf, but if you're thinking about starting meditation, and are looking for practical hints, I'll be glad to share what I know", i.e. if they then admit to having reasons of their own, we can get down to a real discussion of the Dharma - whether in Buddhist terms or in Christian ones or in Atheist or Mystical or whatever terms depends on the circumstances.
If they don't have any personal reasons for asking, it becomes evident at that point, and we can discuss something of mutual interest instead.
Cheers,
Florian
P.S. there's an old forum thread
"What do you tell them"which revolves around this. (I don't want to appear as a spoilsport saying, "read the archives" - that was a very good discussion back then.)