There is no explicit regulation against continuing to use halogen for headlights. It would be difficult to pass the current photometric requirements but not impossible. Halogen has a much more predictable & smooth spectra. The CRI is nearly 100.

LEDs make a lot of sense from an engineering and economics perspective, and that's why we must suffer them. It takes a special kind of leader to burn massive piles of cash/opportunity in order to protect a qualitative thing like how comfortable it is to drive at night. There aren't many CEOs who would be moved much by spectrograms of the headlamps in their cars.