(Hom. xx. 1) But why did he go on baptizing now? Because, had he left off, it might have been attributed to envy or anger: whereas, continuing to baptize, he got no glory for himself, but sent hearers to Christ. And he was better able to do this service, than were Christ's own disciples; his testimony being so free from suspicion, and his reputation with the people so much higher than theirs. He therefore continued to baptize, that he might not increase the envy felt by his disciples against our Lord's baptism. Indeed, the reason, I think, why John's death was permitted, and, in his room, Christ made the great preacher, was, that the people might transfer their affections wholly to Christ, and no longer be divided between the two. For the disciples of John did become so envious of Christ's disciples, and even of Christ Himself, that when they saw the latter baptizing, they threw contempt upon their baptism, as being inferior to that of John's; And there arose a question from some of John's disciples with the Jews about purifying. That it was they who began the dispute, and not the Jews, the Evangelist implies by saying, that there arose a question from John's disciples, whereas he might have said, The Jews put forth a question.
Source: Catena Aurea by Aquinas