To quote Wikipedia: "Like most Slovene ethnic territory, Maribor was under Habsburg rule until 1918"; "The Slovene name Maribor is an artificial Slovenized creation, coined by Stanko Vraz in 1836. [...] The name Maribor was accepted among Slovenes only 25 years later".
@MayaB I get your point, but non-native names have a habit of sticking around in English and other languages. Just because in this case it is not common to refer to Maribor as Marburg (or Ljubljana as Laibach), it will still happen. Neighbouring Croatia makes an excellent case for it, as its two most well-known coastal cities (Split and Dubrovnik) are still very commonly referred to by their Italian names Spalatro and Ragusa.