The phrase ‘kingdom of God’ occurs regularly in the Evangelists’ recollection of Jesus’ words—thirteen times in Mark, another nine times in the material shared by Matthew and Luke (q/Q), a further twenty-eight times in tradition distinctive of Matthew, and a further twelve times in tradition attested only by Luke. It is hardly possible to explain such data other than on the assumption that Jesus was remembered as speaking often on the subject.I wonder if this is really so. Wouldn’t anyone who believed that Christ had risen have interpreted that event as a sign that the kingdom of God had drawn near? Isn’t it an obvious possibility that this phrase was used by the earliest believers to explain the meaning of the visions that some of them were having and that its use was later ascribed to Jesus.
Moreover, suppose that Jesus had been a revolutionary Zealot preaching armed rebellion rather than an apocalyptic preacher and that he referred only very rarely to the kingdom of God. After his crucifixion, some of his followers experienced visions and interpreted their meaning theologically. They would naturally try to remember anything that Jesus had said that related--even tangentially-- to the theological interpretation of his life and death. Those few occasions when Jesus had spoken of the coming kingdom of God would have been discussed thoroughly and would become the focus of the movement's message regardless of how much it figured in Jesus' preaching.
In short, regardless of whether the historical Jesus spoke of "the kingdom of God," a lot, a little, or not at all, we shouldn't be surprised to find it becoming a big part of the traditions concerning his life, because those traditions existed in order to preach the meaning of his death. I think that is very easy to explain the frequency with which the Evangelists used "kingdom of God" regardless of how often Jesus actually did.