I’ll agree that it comes down to the personal vs. group nature of the vehicle. Maybe also who is controlling the vehicle (professional vs. amateur)

You don’t board a motorcycle or a horse. You don’t board a unicycle or bicycle. You don’t board a kayak or conoe or a jetski or a dingy.

You DO board a stagecoach, pulled by horses. You do board a trawler or a fishing vessel.

I don’t board my own van. I do board the intercity bush taxi (the same van with staff and better paint).

Likely all related to use of “board” for ships and devolved from there.