Passage: 20th Century Fox and Paramount Pictures co-financed Titanic, with Paramount handling the North American distribution and Fox handling the international release. They expected Cameron to complete the film for a release on July 2, 1997. The film was to be released on this date "in order to exploit the lucrative summer season ticket sales when blockbuster films usually do better". In April, Cameron said the film's special effects were too complicated and that releasing the film for summer would not be possible. With production delays, Paramount pushed back the release date to December 19, 1997. "This fueled speculation that the film itself was a disaster." A preview screening in Minneapolis on July 14 "generated positive reviews" and "[c]hatter on the internet was responsible for more favorable word of mouth about the [film]". This eventually led to more positive media coverage.The film premiered on November 1, 1997, at the Tokyo International Film Festival, where reaction was described as "tepid" by The New York Times. Positive reviews started to appear back in the United States; the official Hollywood premiere occurred on December 14, 1997, where "the big movie stars who attended the opening were enthusiastically gushing about the film to the world media".

Question: Why did Paramount have to change the release date?
production delays