Why is the UK the only country not to put its name on postage stamps?

Britain invented the postage stamp, so there was no initial reason to identify where it was from. As other countries started issuing their own stamps, the British decided to put a likeness of the current monarch on their stamps to identify where they were from.

https://en.wikipedia.org/wiki/Postage_stamp#History

"Britain is the only country whose postage stamps do not feature the name of the country issuing them. This is because Britain was the first country to produce stamps."

royal.uk/TheRoyalPhilatelicCollection/Britishstamps.aspx no longer exists

Actually this is only valid for stamps which are to be used in international traffic, using the regulations of the Universal Postal Union. Purely local stamps do not have to bear their conutry's name.