"And why, when the disciples of Jesus were baptizing, did not John cease to do so? why did he continue to baptize, and that even until he was led to prison? for to say, 'John also was baptizing in Aenon'; and to add, 'John was not yet cast into prison,' was to declare that until that time he did not cease to baptize. But wherefore did he baptize until then? For he would have made the disciples of Jesus seem more reverend had he desisted when they began. Why then did he baptize?" It was that he might not excite his disciples to even stronger rivalry, and make them more contentious still. For if, although he ten thousand times proclaimed Christ, yielded to Him the chief place, and made himself so much inferior, he still could not persuade them to run to Him; he would, had he added this also, have made them yet more hostile. On this account it was that Christ began to preach more constantly when John was removed. And moreover, I think that the death of John was allowed, and that it happened very quickly, in order that the whole attention of the multitude might be shifted to Christ, and that they might no longer be divided in their opinions concerning the two.
Source: Homily on the Gospel of John 29