The correct answer to this open question is the following.
The right of legation has been honored by nations for thousands of years for the following reason.
The right of legation is the right that every country in the world has to send diplomats to other countries as well to receive diplomats from other nations. So this has been honored by nations in all these years because is the way a country can have a good relationship and trade tides with other nations. It is the way to be amicable in diplomatic terms and send or receive help when need it.