It wasn't just SPICY MYSTERY STORIES and SPICY ADVENTURE STORIES which began publishing sf around 1940.  In addition to the launch of several new sf magazines, other pulps which had never published science fiction before, such as Fiction House's ACTION STORIES, began running it and featuring it on the covers.  I'm not sure as to what caused this surge of interest, but apparently more than one publisher felt that the way to make a buck was to run some of that Buck Rogers stuff.  Oddly, authors such as Heinlein or Asimov never appeared in the pages of SPICY MYSTERY STORIES in its sf phase...