Why most measurements equipment uses 10 MHz as reference?
There are a lot of technical reasons why SC cut Xtals are  1k more stable than AT cut in ovens at the  turnover temperature in a OCXO and why 10MHz is preferred for phase noise over 100MHz and why 10MHz was more convenient than say 12 or 20 or  say 1MHz for size reasons.  

But it is true that 10MHz OCXO’s are the most common frequency and with 1E^-12 stability per day these can be calibrated against nuclear-based time and frequency references.