Why are disc wheels not used in the front on professional track sprint races?

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).

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 it's a little more skittery. A disc in match sprint c/would be dangerous. UCI recognizes this.