Why are they used only in the back if there is no risk of winds at all?
Sprint often requires quick direction changes. With a quick turn of the wheel a disc becomes a rudder in the wind and can behave unexpectedly. Even in TT, many riders will elect something other than a disc in the front simply because front discs are a little more skittery. A disc in match sprint c/would be dangerous. The UCI recognizes this and only allows rear disc in match sprint.
Because it's not permitted under UCI regulations.
Rule 1.3.018 applies, which includes this section:
In track competition, including motor-pacing, the use of a front disc wheel is only permitted in the specialities against the clock.
Disc wheels are commonly used in the sprint qualifying round (i.e. the flying 200 metre TT).
The natural follow-on question is : Why do UCI regulations allow rear disc wheels, but not front disc wheels?
I can't answer for the UCI technical commission but to guess it's because the handling characteristics of a front disc wheel makes them less suitable for rapid response to steering input, a trait desirable in mass start and match racing (ie, it's a safety consideration). Using a disc on the rear doesn't affect steering response. The difference (slowness) in steering response of a front disc was my experience when using them on the track. Apart from individual timed events I used them during team pursuits.
As to there being no wind, while that's true in an absolute sense, it doesn't mean a bike and rider experiences zero yaw at the track. You experience wind yaw when in the turns, and when entering a turn a rider experiences a change in yaw angle which also contributes to handling issues with a front disc. On average the yaw experienced on the track is a few degrees. Timed events are ridden under controlled and predictable conditions with inherently less risk than match and mass start races.