Trekkies take note! The answer to your question on "Why haven't they developed a "Star Trek Cell Phone" has just been answered.
Today, Sona Mobile and Viacom Consumer Products announced they've been developing the first officially licensed Star Trek-based cell phone, dubbed "Star Trek Communicator Phone".
According to the announcement...
Themed after Star Trek communicator devices popularized by characters on the science fiction television and movie series, the special edition Star Trek Communicator Phone offers Trekkers and fans of cool mobile technology the ability to play a multi-player, online Star Trek game, stream real-time video, and surf the Internet as well as access Star Trek ring tones, wallpapers, news, information, and other fan activities.The new phone is expected to start selling on September 30, 2005.
The Star Trek Communicator Phone is powered by Sona Mobile's Sona Wireless Platform™ (SWP), which offers multi-threading capability for running several applications concurrently. The SWP makes it possible for end-users to stream video clips while simultaneously text messaging a friend or accessing information on the Internet, without negatively impacting device performance.
Sorry, can't find an image.