There are all sorts of clans with different beliefs and cultures that form the Mandalorians, even if they all share the same strong affinity to combat, armor, beskar, etc... It is possible that Din Djarin came from a culture that has the surname before the first name, like in several asian cultures in our world. I mean, I don't think Din Grogu sounds better than Grogu Djarin, and it's not like most people would change how they call him anyway (a lot still call him Baby Yoda to this day), so I cannot think of a specific reason they'd go with that except to reflect different naming conventions for different clans.
EDIT : take it for what it's worth but the Wookieepedia page calls him Djarin all the time while recounting his adventures, only uses Din to formally name him fully.
