Because say you fire a photon from an emitter to some target and you want to know how much time elapsed for that flight, but how would you find out? The target will have to communicate to the observer (you in this case) when it received the photon, but that communication will require.. more photons, and a trip back to you. If you're colocated with the emitter then it's a round-trip, else you need a photon from the emitter and the target, as well as the one from the emitter to the target, and this amounts to a round-trip anyways.

Therefore you can't measure the speed of light in any one direction. You can only measure the round-trip time of flight (e.g., if you have the detector at the emitter and use a mirror).