The answer to your question "what has changed to make things more favorable for moose" might just be pretty simple.  Sometimes, people who died decades ago used to tell stories about it.  When a moose came into the country it was hunted down, killed and eaten.  

Animals like moose were not able to gain a foothold.  Their population cycles would be very, very long, and dependent on a collapse in predator populations, including humans. With the arrival of imported food sources and game management, populations are protected during their down cycles, and thus have a good foothold for the upswing; evening out the "see-saw" of nature.