Because people love and trust Sparkfun, other than that, I have no idea. Unless you have something small (say under 4 sq in) your likely cheaper getting someone else to make it. I use them every now and then when I need a breakout board for -that-fscking-small-weird-footprint-device-I'm-tryingout- usually this means I can get 4 breakout boards in less than 3sq in, there's no point paying someone else (or wasting a panel slot) to make 3sqin of something if I only need one set.
fwiw, I do like sparkfun itself very much. It's just their board service that seems to make no sense.
That small weird footprint device is exactly the exception I've mentioned though where you would want a pcb even for a one off.
I do want to comment that if you need that breakout, there's probably 20 other people on here who would be interested but don't want to take the time with Eagle or the expense of $17.50 to make 3. So, for something as general-purpose as a breakout, why not have a 24 square inch panel of them made at Olimex or custompcb.com or some such and sell the extras on your site? Even if you do end up with coasters, it's really not that much more than the batchPCB price for a much smaller amount.