Here's my (conspiracy) theory: If you can find it, go read the owner's manual that came with your bike. Almost certainly it will say something along the lines of "never ride your bike at night." Why? Liability. They don't want any.

In 1993, there was a case, Johnson vs. Derby Cycles, involving a cyclist who was riding at night on a bike that had only the reflectors that came with it. He was hit by a car and severely injured. He sued the manufacturer of the bicycle, claiming that he had been misled into believing that the reflectors made it safe to ride at night. The plaintiff was awarded $7 million. You can read about the case here:
http://www.johnforester.com/Consult/GreenJM/derby.htm

This decision created a big problem for bicycle manufacturers. They are legally required to put reflectors on every bike they make, but there is now caselaw precedent that selling a bike with reflectors but not lights is negligent. Their solution is to make sure that every new bike they sell includes a directive that the bike is not to be used at night.

My bet is that the helmet manufacturers want no part of this litigation lottery, and their lawyers have advised them that reflectors on the helmet would expose them to liability.

This case is also why you can't buy a bike with anything more than the bare minimum legally required reflectors. It would be great to have a frame painted in reflective paint, but no manufacturer dares.

I'm building up a frame right now for a new inclement weather bike, and I just finished wrapping the frame in some DOT-C2 conspicuity tape I got off Ebay. It looks fabulous!