The formal definition of the Ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between those conductors a force equal to 2 x 10-7 newtons per metre of length.

Why does this number appear here? Why isn't it any other number or defined in terms of electron charges or the speed of light or something fundamental?