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 and the NWS will continue to forecast icing and turbulence in AIRMETs and SIGMETs.
Decoding TAF reports
I would say it is because there are other weather charts that cover temperature, wind aloft , etc