Bald caps are trickier than some might think. It can often look really bad on an actor, and the last thing a filmmaker wants is something that looks unnatural.
Paramount attempted a baldcap Deltan in ST III, on Excelsior. It looks like someone wearing a baldcap.

http://www.reocities.com/therinofandor/ufp/2.jpg
I explored the Deltans-with-hair problem here:
http://therinofandor.blogspot.com.au/2006/11/truth-about-efrosians-pard_116269248336471480.html