The principal cast members of Citizen Kane were not Hollywood actors. Rather, they were theatrically trained actors Welles had assembled many years before with his partner and mentor, John Houseman. In 1934, when Welles and Houseman met, Houseman was thirty-three years old and was already highly respected in the theatrical world as an actor, director, and producer. Welles and Houseman produced plays together through the Federal Theatre Project, a program formed under the Works Project Administration (WPA) to provide employment in the cultural arena. Their first project was a daring adaptation of Macbeth—they used black actors and staged it as a voodoo-themed production. In 1937, they both resigned from the Federal Theatre Project after one of their plays, The Cradle Will Rock, was closed down by federal agents because of its leftist politics.
Shortly thereafter, Welles and Houseman founded the Mercury Theatre. The group consisted of many of the elite theater actors of that time, and they were known as the Mercury Players. Eventually, the Mercury Players included actors who went on to make a significant impact in film, such as Joseph Cotten, Everett Sloane, Ruth Warrick, and Agnes Moorehead. Welles and Houseman’s ambition for the Mercury Theatre was to stage the classic plays “with their original speed and violence,” as Houseman once said. Their first play, a modern adaptation of Julius Caesar set against the backdrop of Nazi Germany, was a great success. In 1938, Welles, who had already been working very successfully in radio, formed Mercury Theatre on the Air, a weekly radio broadcast starring his Mercury Players. At this time, radio focused more on drama than music, and the Mercury Players, with their theatrical talents, were extraordinarily well-suited for the medium. Their most famous performance was the 1938 Halloween eve broadcast of War of the Worlds.
The Mercury Players soon moved beyond the limitations of radio. Shortly after the War of the Worlds broadcast, Welles accepted his Hollywood contract and moved west. At first, he flew back to New York to do his weekly radio broadcasts, but he eventually brought the Mercury Players to Los Angeles. Welles and the Mercury Players continued with the weekly Mercury Theatre on the Air radio broadcasts as Welles worked to develop a project for RKO Studios. Welles was anxious to cast his theatrical group in his first movie, but the length of time it was taking for Welles to settle on a project took a financial toll on the actors. Some had to take other jobs. A few got roles in other films, which upset Welles—he’d wanted their debuts to be in his film. The stress ultimately led to a blowup between Welles and Houseman, and their partnership ended.
If the RKO executives had not signed away so much control to Welles, the studio undoubtedly would have objected to Welles’s plan to cast his Mercury Players in the key roles in Citizen Kane. Then, as now, the idea of casting unknowns in a major picture met a great deal of resistance. In fact, Welles maintained that the reason an earlier project he had been developing never got off the ground was that the studio was unwilling to let him cast Lucille Ball, who at that point had never starred in a major picture. In Citizen Kane, however, Welles was able to cast his unknown Mercury Players, and much of the success of the film stems from how well their theatrical training worked within the dramatic framework of the movie. The fact that they were unknowns actually may have contributed to their effectiveness, since more recognizable players may have distracted viewers from the story.
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.