The reason for this I heard by Microsoft was to stop having to explain to non technical people that Azure AD doesn't have feature parity with a real Active Directory.
As a Portuguese native Microsoft FTE, I found the name change jarring. “Entra” means “Enter”, with a subtext of permissiveness that seems overly positive for an authentication mechanism. But then again, Continental Portuguese is seldom considered as a semantic idiom, and Brazilian Portuguese (or even Spanish) tend to have a lot more influence in branding everywhere on the planet.