I can only imagine this is borne out of "ways we saw marijuana transported while it was still illegal."
Completely spitballing this one, but I can imagine that a bike might be argued to not be a safe way to transport drugs or any sort of high-value delivery, since they are less easily secured and more easily stolen than a locked motor vehicle when it's parked outside an apartment building (for example).
^ This. The California government likes to regulate away things it thinks will be dangerous, or that it's citizens will be too ignorant to avoid doing. Honestly I'm kinda surprised they didn't specify armored car transport as the only acceptable option.