Because the craters on the Moon were not formed continuously, and not by ordinary lightning, according to the model. The Moon was captured by the Earth, and during that process the Moon and Earth interacted electrically and massive discharges occurred, facilitating the capture process.
And I should add that not all craters are from impacts, but some are.

And that the model isn't "cutting the cake" is the general feeling, although exploring some facets can really help us understand some strange findings. Like the cratering that is visible throughout the solar system and I'm still struggling to understand how the solar wind/heliosphere works and what powers the Van Allen belts and how some comets get their double (or even broken) tails to name a few things.
Cheers.