Why are they used only in the back if there is no risk of winds at all?
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).