There are a couple reasons why December releases don't dominate the all-time box office. Studios were deathly afraid of opening big tentpoles during this time period.
1. Lower Opening Weekend. Doesn't sound as good press wise. Holiday shopping tends to force box office goers to take a "wait-and-see" approach to films.
2. Reception has bigger impact. Due to a lower opening weekend, in order to "match" the expected gross from a summer release, you need good reviews in rder for legs to carry through the holidays. Now as we have seen really recently, the time period it takes for this to manifest has decreased. But before, you couldn't get away with having bad reviews during December. You lower your OW and then WOM kills you. Suddenly, you're left with much less than you would have gotten had you just opened in May.
3. Weather. This is a factor often overlooked. The unpredictability of blizzards and icy roads can really hurt box office grosses. Sun time fun time as they say. You can block the road to a movie theater with snow, you cant melt the building