Google found this for me:

What about higher-wattage bulbs for my DOT-spec headlamps?
One word: DON'T!
If your headlamps have plastic lenses and/or plastic reflector housings, you
mustn't use overwattage bulbs, as this can create serious fire hazards and
cause extensive (and expensive) damage to the headlamp assemblies and wiring
in short order.
Even if your headlamps use glass lenses and metal reflector housings, you
still can't safely use these bulbs because the wiring in DOT-spec headlamp
systems is not capable of handling the level of current these bulbs will
draw. If you disregard this and do it anyhow, you create several serious
fire hazards in the engine compartment and, more worrisome, in the passenger
compartment at the headlamp switch. This problem could be circumvented with
the installation of an extra-heavy-duty relay and heavy wiring, but the
wiring is only as robust as its highest-resistance section; the 9004/HB1,
9005/HB3, 9006/HB4, and 9007/HB5 bulbs used in DOT-spec headlamp systems all
have extremely small electrical contacts. Go ahead and have a look; they're
really spindly! These contacts get extremely hot even under normal (45 to 65
watt) loads. They get dangerously hot under higher (80w, 100w, etc.) loads.
When you go much above 65 watts, these contacts become the point of maximum
resistance. Things start melting and burning, and due to the voltage drop
involved with such resistance, you will not achieve any improvement in your
headlamps' ability to light-up the night. No reputable bulb manufacturer
makes these overwattage DOT bulbs for that reason.
The ones you may find are third-rate junk that not only will not last very
long, but also are made in factories with little or no quality control and
do not tend to be made to the extremely precise filament-placement
specifications demanded by today's advanced headlamp designs for proper lamp
performance.
Even if you could find (nonexistent) overwattage bulbs in these formats with
oversized contacts and ceramic bases, make your wiring adequate to handle
the extra load, and be sure your headlamps contain no plastic, you *still*
would be wasting your time and money to use the overwattage bulbs, because
the optics and beam pattern specified in such headlamps by the DOT does not
respond to overwattage bulbs, except to cause massive dazzle to oncoming
traffic and drivers in front of you via the rear view mirrors, and to cause
extreme backglare in fog, rain or snow. Any way you slice it, overwattage
bulbs are a bad deal all around. You'll do well to avoid them.

My guess is focusing.

Unless you can find a reflector assembly that keeps the brightness
pattern the same as HID lights (sharply focused so as to produce a sharp
line between "bright" and "dim") then such high-power incandescents will
simply be too bright for other drivers.