可能因为历史原因，对电流的发现研究和应用早于发现电子。我猜的

因为库伦不是基本单位

定义安培为基本单位，定义而已。

定义。

The SI was previously defined in terms of seven base units and derived units defined as products of powers of the base units. The seven base units were chosen for historical reasons, and were, by convention, regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. This role for the base units continues in the present SI even though the SI itself is now defined in terms of the defining constants above.
从国际计量组织摘抄过来的，选择这7个基本单位是历史原因。虽然现在的国际单位制已经更改为基于7个基本常数推导，不过7个基本单位的功能还是顺延下去。

另外由于基本单位现在都是由基本常数导出的，所以现在的1安培的定义是

或者
其中e为基本电荷常数1.602 176 634 x 10–19（C），
Cs 为铯原子钟振荡评率常数9 192 631 770 （Hz）