Remember the merch fiasco they had after Frozen came out in 2013? This isn't new for them. They were so ill-prepared of the massive success of the film that merchandise was in and out of stores for close to a year because of how long it takes to manufacture newly ordered merch.
Josh Gad just told a story on Kimmel about how his daughter wanted to be Elsa the year after the film came out, but dresses were sold out everywhere. Gad asked Iger if there was any he could get and Iger responded by saying he just got one of the last one's from Aulani shipped for his granddaughter.
Point is, Disney plays it cautious with new releases on the merch side because there's always the chance of it being a huge hit, but you never want to order a ton in expectation of a hit and be left with a ton of unsold merch. Situations like this are a tough needle to thread.