Orson Welles dominates Citizen Kane. He produced, directed, and starred in it, and his overpowering presence both on and off screen has often overshadowed the fact that the film was actually the result of a successful collaboration between some of the greatest minds in Hollywood at that time. The greatest controversy is over who wrote the script, and this battle has colored much of the discussion of this movie over the years.
Before making Citizen Kane, Welles had been accustomed to taking full credit for works that were often collaborations. When Welles worked with the Mercury Theatre, the Mercury Theatre on the Air scripts were generally credited as studio productions—until the great success of the War of the Worlds radio broadcast. The broadcast generated so much publicity that Welles decided to take full authorship credit, even though it had actually been written by studio writer Howard Koch. The Mercury Theatre continued to encourage Welles to take credit for productions, believing that his name and reputation would bring good publicity. Welles’s reputation as a theatrical genius had been growing since his adolescence, and the Mercury Theatre was more than happy to take advantage of it.
Welles eventually began to buy into his own publicity, and he conveniently disregarded the fact that he was not the sole creative genius behind his troupe’s endeavors. His ego wasn’t welcome when he went to Hollywood to work on Citizen Kane, and, not surprisingly, Hollywood wasn’t willing to give Welles credit he didn’t deserve. Welles met huge opposition when he tried to take full credit for creating Citizen Kane.
Although he played a key role in writing Citizen Kane, Welles did not create the script single-handedly. Much evidence suggests that the original idea for Citizen Kane came from Herman Mankiewicz, a battle-hardened Hollywood scriptwriter. Mankiewicz was well acquainted with William Randolph Hearst, having spent a great deal of time at Hearst’s ranch in San Simeon. Charles Foster Kane, the protagonist of the film, resembles Hearst in many specific, personal ways, and such information could have come only from insider knowledge of Hearst's life, which Welles did not have. Welles did play an important role in creating the script, and few critics doubt that he drew from his personal life, in the same way that Mankiewicz drew from Hearst’s life, to flesh out the character of Charles Foster Kane. Welles, however, was certainly not the only person responsible for the script’s creation.
Mankiewicz collaborated with John Houseman, Welles’s partner at the Mercury Theater, on the initial draft of Citizen Kane. Both Mankiewicz and Houseman wanted writing credit on the final version, but Welles refused. Houseman gave up when Welles dug in his heels, but Mankiewicz had the power of the Hollywood writers’ union behind him. He threatened Welles with legal action in order to be listed as a writer, and Welles yielded. On Oscar night, Citizen Kane won the award for Best Original Screenplay, and this was the only Oscar either Welles or Mankiewicz ever received.
I couldn't figure it until having watched it again; but it turns out that there is enough to know why his mother abandoned him. Considering the times, the situation and the few actions of Moorehead as his mother, I think I can safely assume that Kane's birth father is abusive, likely alcoholic; and his mother gave him up to insure he is safe from his father and the especially the people that they both became, the life they both sunk into; essentially her acquiescence.