Because the law in question is civil, not criminal, and because the violation was found by an executive branch administrative law judge, not by a court of general jurisdiction in even a civil lawsuit.

Regulators implicitly allowed the ambiguity to persist, and are now attempting to reframe or correct it retroactively.

It’s possible there were political or practical considerations — for example, a belief that successful autonomous driving would make future regulation easier, or at least postpone difficult legislative debates.

We can’t know what the internal reasoning was, but the long delay suggests more than simple oversight.