January 3, 1941 Martin Bormann issued a circular to all public offices which declared Fraktur and its corollary, the Sütterlin-based handwriting, to be "Judenlettern" [Jewish letters] and prohibited their further use.
One possible truer reason for the ban was that Antiqua would be more legible to those living in the occupied areas. Indeed, readers outside German-speaking countries were largely unfamiliar with Fraktur typefaces which were no longer well known outside the countries of German language. However, the Nazis had been printing books, newspapers and miscellaneous texts destined for abroad in Antiqua for a long time.
It is more likely that the reason for the reversal was Adolf Hitler's dislike for the Fraktur typeface, as demonstrated by a declaration made in the Reichstag in 1934
"... In a hundred years, our language will be the European language. The nations of the east, the north and the west will, to communicate with us, learn our language. The prerequisite for this: The script called Gothic is replaced by the script we have called Latin so far..."