We can glean some reasons from the text of the scriptures as to why John did not follow Jesus. First, John denied that he was Elijah (John 1:21), but Jesus indicated that John actually was supposed to act as that prophet:

Matthew 17:12

> I tell you that Eli′jah has already come, and they did not know him,
> but did to him whatever they pleased. So also the Son of man will
> suffer at their hands.”

John had a stricter external discipline that Jesus. He taught his disciples to fast and refrain from wine. Jesus did not.

Matthew 9:14

> Then the disciples of John came to him, saying, “Why do we and the
> Pharisees fast, but your disciples do not fast?”

John was outspoken politically on the issue of Herod Antipas' marriage, which was unlawful according to the Torah. Jesus is not recorded as saying anything about this.

Matthew 14 

>  Herod had seized John and bound him and put him in prison, for the
> sake of Hero′di-as, his brother Philip’s wife; 4 because John said to
> him, “It is not lawful for you to have her.”

Despite his spiritual experiences confirming Jesus' identity at the Jordan, John seems to have doubted whether Jesus was actually the messiah:

Matthew 11 

> 2 John... sent word by his disciples 3 and said to him, “Are you he
> who is to come, or shall we look for another?” 4 And Jesus answered
> them, “Go and tell John what you hear and see... and blessed is he who takes no offense at me.”

Based on these scriptures, we can identify four reasons why John refrained from joining Jesus as his disciple: 1) He did not agree with Jesus that he was play the role of Elijah. 2) He taught a more ascetic discipline than Jesus did. 3) He felt it was important to speak out against Herod Antipas while Jesus did not. 4) He doubted that Jesus was "he who is do come," apparently because he took offense an some aspect of Jesus' teaching or person. 

We may speculate about other reasons: John, as Jesus' older cousin, was in the archetypal position of the biblical elder brother who has to subordinate himself to the younger brother. This is never easy, and John's pride may have played a role in his "taking offense" at Jesus. In addition, Jesus recruited his first disciples from the ranks of John's followers (Jn 1). John may have felt that Jesus was a "sheep-stealer." Finally, there is a tradition among the [Mandaeans](https://www.newworldencyclopedia.org/entry/Mandaeanism) (a contemporary Gnostic group that traces it roots back to John the Baptist) that Jesus was supposed to remain with John and support him. This, of course, contradicts the Biblical account, but there may be a kernel of history in it that could help explain why John did not follow Jesus. 

There above factors may have all contributed to the fact that John did not follow Jesus.