Because that represents the speed of light. Once the decision was made to use c as the reference instead of a bar of iridium-platinum, they defined in terms of the best measurement of c, and then defined the value.
It is true that the oringinal meter was defined to make the distance from equator to pole equal to 10,000 km. But they incorrectly accounted for the oblateness of the earth, so that ended up being incorrect. The wavelength, and then c definitions were adopted because the meter could be determined more accurately and precisely, just as the definition of the second was shifted to an atomic definition from being some fraction of the earth's rotation under some set of conditions. You also have the advantage that anybody with sufficient technology can realize the standard themselves, without having to go to Paris to compare with the world standard, or have the BIPM manufacture a new one for you.