1) You should not WANT to wear a watch of any kind during a race. Not only is it distracting but consumes energy and takes away from the authenticity of racing.
2) Two way interference of radio signals and frequency. Remote sensing of timing mats to "chip" can be interfered with as well as watch to satellite due to the signals and frequencies of the mats.
3) Can you imagine the dumb 16 year olds and their coaches complaining about GPS showing the 2 mile marker "in the woods", for instance, is off? Commercial GPS has a 15m/s^2 margin of error. GPS can pinpoint your precise location from a theoretical circle encircling your device signal which is actually the second draw. First it theoretically draws a larger triangle to enclose your device signal then looks for a place to encircle for your precise location. But if the GPS cant draw a precise circle because of a tree or failed signal it will default to a nearest location in the larger triangle which could be "out of the woods". To that extenr, timers, governing bodies and referees want nothing to do with it.