Usually, singles need to have horizontal dropouts so you can take the chain slack by adjusting the rear axle position.
That means that any brake that is attached to the frame will "go out of position" when you adjust the rear axle position. That is, by the way, the reason why some horizontal dropouts are not quite horizontal, but diagonal: to be near-perpendicular to the seat stays where rim brakes are attached, so that wheel movement is "tangential" to the brakes.
Unfortunately, with regular disc brakes, this adjustment would totally misalign the brake pads relative to the disk, if the adjustment is possible at all.
Although there are some bikes with grooves for the disk caliper, where you can "slide" the caliper into, it's usually not worth the trouble, because chain slack has to be corrected very often, and caliper setup is relatively laborious.
Also, it would be possible to take the slack with an eccentric bottom bracket, but that is even more esoteric.
Next to that, some single-speeds will come with a flip-flop hub. A flip-flop hub and a disc brake doesn't go well together.
Hope this helps.