I agree, it would be nice to have a single standard, but as far as I am concerned, you can blame the IARU for that.
By 1981, there were millions of radios out in the public and the vast majority of them (CB's), used -67dBM/100uV/+46dBu for S9. Even amateur radio makers had different "standards". Kenwood, Yaesu, Collins, Swan, National, etc. all had different settings for S9.
In 1981, the IARU put forth a "recommendation" to use the "Collins Standard" (-73dBm = S9) for freqs. below 30Mhz. Since then Full HF rigs pretty much use that standard.
However, even current CB's and 10 meter radio manufacturers, with the exception of CPI, continue to use -67dBm for S9 as they have for the last 50 years.
So instead of doing the smart thing and adopting a standard that matched 90%+ of the existing radios at the time, the IARU chose something different...