"Nissan rebadged the Datsuns for the same reason that Mitsubishi rebadged themselves from Colt in some markets - worldwide consistency in marketing.".
It was Datsun in Finland too, the first European country to import them (1962) also the first Toyotas came to Finland. I had a 1967 Toyota I bought as second hand, The joke then was "when will you get a real Yota".
For the name change, why not use the Wiki. (trying and trying to teach you commentards).
"Datsun is an automobile brand owned by Nissan. Datsun's original production run began in 1931. From 1958 to 1986, only vehicles exported by Nissan were identified as Datsun. By 1986 Nissan had phased out the Datsun name, but re-launched it in June 2013 as the brand for low-cost vehicles manufactured for emerging markets..........".
Japanese cars came as a shock for car makers around the world, cheep, reliable no fuss cars. It took some time for European producers to recover. The Brits never did although I suppose there was other deep reasons too.