First of all, Jesus never said that John the Baptist "isn't found in the kingdom" as the O.P. statement claims.
Some take "the kingdom of heaven" to mean the place where only 144,000 humans will be found, and that there is an earthly kingdom where a few billion people will live forever (never getting to heaven), One answer does that grave injustice to what Jesus actually said about John.
That answer claims that "by anointing them [the disciples] with holy spirit, the total number of those to be redeemed/purchased from mankind total one hundred and forty four thousand". That view seems to be that John the Baptist had died before that arrangement for a kingdom was inaugurated by Jesus, so that all faithful ones who died prior to Jesus' death would await the still future Day of Resurrection on the earth. They would be raised with physical bodies to work their way (over 1,000 years) to eventual everlasting life on earth. None of those billions will ever see the "kingdom of heaven". The 144,000 are "greater than" all those resurrected to live in the earthly kingdom, with those in heaven ruling over them - hence, superior to them.
It is vitally important to mention this if your question is to be properly answered. Any who labour under a misapprehension, that John will never be found in the heavenly kingdom, which is what they mean when they said he "isn't found in the kingdom", will prevent understanding of my answer.
John was anointed with the Holy Spirit as Luke 1:13-17 makes clear. In part it states:
"For he shall be great in the sight of the Lord... and he shall be filled with the Holy Ghost, even from his mother's womb." (A.V.)
So, what did Jesus mean? The preparatory work of John was about to close. Antagonism to Jesus' ministry was open now and there was a turning point reached. This is what Jesus was announcing in Matthew chapter 11, as explained in this book:
"What was the response to the light of that glory of the Messiah which
had been testified, and which had shone, throughout their land? The
rulers and the nation of Israel totally rejected the light both of
Messiah and the kingdom. And why? Because they loved darkness rather
than light. This appears throughout the crucial passage in Matthew
chapter 11. This is the turning point in the counsel of God toward
Israel as a nation. A climax has been reached. At this point testimony
has been rendered to the utmost both as to the person and work of
Messiah, and the heavenly and spiritual nature of the kingdom." (The
Evangel According to Matthew: an exposition p22, John Metcalfe, 2011)
John had, by the Holy Spirit, played his role in that, by preparing a people who, repentant and baptised, would receive Christ as Messiah. He had faithfully fulfilled Malachi 3:1 but would not live to see Christ bring in the New Covenant with his resurrection and ascension. Those he had prepared would. As this other book explains:
"John's ministry will cause men to turn from darkness and to face the
Light... [John's] first preaching was in the wilderness. Then there
was baptism in Jordan, with confession of sins. Forgiveness of sins is
not mentioned. Confession of sins is entailed in receiving John's
words and being baptised as a result... Under John's ministry, sins
are exposed; sins are admitted; sins are confessed. No more." (The
Beginning of the Gospel, page 39, Nigel Johnstone, 2012)
"The ministry of John the Baptist, the preparative messenger, is a
ministry that prepares the heart and mind for the coming of Christ to
the soul, as conveyed in the gospel. It, itself, does not convey
Christ. It prepares for that event. And if the preparation is not
received, nor will Christ be received." (Ibid. page 46)
To be prepared to receive the gospel of Christ is essential. Without it, the good seed of the kingdom will not be in rich, deep soil, so as to take root and grow. See Mark 4:1-20. Those properly prepared see Christ as the Light of the world. John's ministry took Christ's people so far, preparing them. It was what followed that was greater than anything John could do. John pointed to the Son, but the Son reveals the Father. After John's death, the mystery of the kingdom began to be opened up, and after Jesus' resurrection, the Holy Spirit was sent to empower all those who had been prepared by John to receive Christ the King of that kingdom. The power of the kingdom began to explode and expand. But up till John's death, there was the light of a new dawn in a dark land, to which John pointed, preparing people for the Light of the world. Thus, those who then saw and believed what John never got to see, could be said to be "greater", for their faith grew on the basis of the resurrection and ascension. Their testimony was therefore greater, as it dealt with the forgiveness of sins through faith in the risen Christ.