Blackletter typefaces were exclusively used in German and German-friendly countries for several centuries. They were often commonly and errornously called "Fraktur" in German, but the correct name was gebrochene Schrift while Fraktur was only one (but the most used) typeface.

Wikipedia Commons, Public Domain
Wikipedia’s take on the topic is roughly this:
On January 3, 1941, the National Socialist German Workers' Party, or more precisely Martin Bormann, issued a circular to all public offices which declared Fraktur to be Jewish letters and prohibited their further use. German historian Albert Kapr has speculated that the régime had realized that Fraktur would inhibit communication in the territories occupied during World War II.
But this seems a strange reason for people that valued their own ideology over everything else. Especially the allegation of "Jewish letters" strikes me as odd, seeing how Fraktur originates in the early 16th century and was commissioned by Maximilian I, an emperor that kicked the Jews out of some regions in 1496 and in 1509 passed the "Imperial Confiscation Mandate" which ordered the destruction of all Jewish literature apart from the Bible. Seems pretty in line with what the National Socialist German Workers’ Party did.
So why was Fraktur abolished, then?