They are more recent.

There were hundreds of units of distance a long time ago, when there wasn't a lot of communication or cooperation between nations - or sometimes even between cities in a nation (see: China, Rome). As humans spent more time interacting and cooperating, we standardized our measurements; but the US stubbornly refuses to change from the system it uses.

However, electricity was discovered and measured recently enough that competing units of measurement never got established enough for there to be significant argument over which units to use.