The short answer is: Because Gourmet Magazine says so.
Long Answer: At the very least, you should know what a microplane is. Avoid embarrassing yourself at a dinner party and get familiar. But let's put aside the issue of protecting your fragile reputation. A microplane is a very handy kitchen tool. It grates or peels your fruits, vegetables, cheeses, roots, etc... all with ease! It's easy to clean and store. I swear by it when a recipe calls for the zest of a lemon, or grated fresh ginger, or even shaved chocolate. You can get one for under $15.
But what's really interesting to me is that Gourmet Magazine would have you think the microplane has gone mainstream. I was reading the old Thanksgiving 2007 issue the other day, and I found a recipe with an ingredients list that called for something to be grated on a microplane. Gourmet just assumes you know what it is as they go about their business of sharing the recipe. They don't pause for a second to help you poor unfortunate (naive, ignorant, uneducated) souls who aren't familiar with the term. Or maybe they figure it's fine to say microplane in the ingredients list because you'll look it up in the dictionary. And speaking of the dictionary, it's not officially added, though it does have an entry in Wikipedia and it's been reviewed all over the internet including on non-food sites, to rave reviews.
It's basically the Kleenex of graters, the only one with a household name. I mean, who asks for a face tissue when you could ask for Kleenex instead?