Why are min and max temperatures (TN TX) not published in TAFs in the United States?
This isn't much of an explanation, but the National Weather Service says:


  The international TAF also contains forecast temperature, icing, and
  turbulence. These three elements are not included in National Weather
  Service (NWS) prepared TAFs. The U.S. has no requirement to forecast
  temperatures in an aerodrome forecast


That sounds to me like "we don't think it's worth it". There are various temperature forecasts on the NWS aviation site (icing level, surface temperatures, graphical area forecast) but personally speaking, I get low/high temperature information from regular, non-aviation forecasts. You can get those forecasts from the NWS site too, if that's what you mean by "official".