David,
That's a good question...
First, I should point out that DSLR's work just as well with prime lenses as they do with zoom lenses. If you look at a camera dealer's selection, you'll usually see DSLR's offered in zoom lens kits because zoom lenses are always more popular with consumers. This is due to zoom lens' convenience, not because they work any better or worse than prime lenses.
The problem with focusing, is due more to the fact that most modern lenses have auto-focus. Unlike my older Zuiko prime lenses, most modern AF lenses have 1) and AF over-shoot range to let the lens overshoot infinity while focusing (AF lenses work by overshooting the target, then homing in on it by reducing the error each time they overshoot), and 2) the distance markings are usually under a transparent, sealed window. Both of these factors make it very difficult to focus an AF lens at night.
There seem to be two popular solutions. First, shine a bright flashlight on the subject, and then focus manually through the lens. Second, use a wide-angle lens, and manually set the lens at infinity (or, a hyperfocal distance just below infinity), and don't shoot any close-ups.
I usually use #2, although most NP'ers around here seem to prefer #1.
Just my two cents...
Andy