The simple answer is because no one was granted a heavenly reward until after the death and resurrection of Jesus. John the Baptist died before that time. John was not invited to become one of the disciples of Christ, nor one of the apostles, neither did he live until Pentecost to share in the blessings there poured upon the faithful. He belongs to a class described by Paul in Hebrews 11:38-40. He was in a class with Abraham, Isaac and Jacob and all the prophets, of whom the apostle says, “They without us (meaning the faithful followers of Jesus) shall not be made perfect.” He did not receive a heavenly reward because he died before Jesus, but still he was great. 

There is a distinction between the different time periods, or ages, of the Bible. John was the last of the prophets in the Jewish Age. At Jesus’ first advent, that period of special favor to Israel was ending. The apostles and other followers of Jesus lived during the time the gospel message was sent forth, the Gospel Age, which Jesus was inaugurating. Hebrews 10:20 NIV, “…a new and living way opened for us through…his (Jesus’) body”. 

The gospel call is a call to participate in the kingdom of God with a heavenly reward at resurrection. Our Lord Jesus Christ is to be the Great King of the World during that future kingdom. God is presently selecting a “little flock” to be our Lord’s associates to bless the future resurrected world of mankind. 

The first ones to enter this new and living way received the holy spirit at Pentecost. Since then, all who consecrate and are begotten of the holy spirit also travel the living way of following in Jesus’ sacrificial footsteps. If faithful through the trials and testings of the present life, these shall be “kings and priests unto God” and shall reign with Jesus a thousand years for the uplifting and blessing of mankind. (Revelation 1:6; Revelation 20:4.) 

The least one in Christ’s body will have a higher station than John the Baptist or any of the Prophets of the Jewish Age. In the resurrection, all faithful Christians will be spirit beings in heaven. John the Baptist, with all the faithful of the past, will also have great blessings, yet they will be resurrected on the earth. 

Isaiah 1:26 NIV, “I will restore your leaders (faithful prophets) as in days of old, your rulers as at the beginning. Afterward you will be called the City of Righteousness, the Faithful City.” The faithful in past ages will be restored with blessings to human perfection. Acts 3:21 NASB, “(Jesus) whom heaven must receive until the period of restoration of all things about which God spoke by the mouth of His holy prophets from ancient time.”

Additional Resource:
Christian Questions Podcast 
Episode #1302: “How Can I Doubt My Doubts? (Part I)”
Learning the difference between constructive and destructive doubts
(John the Baptist discussion at 00:49:13)
Preview Video
CQ Rewind Show Notes