An interesting observation, Venus, and one with several explanations.
One view would be that the firebrand "end is upon us" John the Baptist either felt disillusioned that his expectations had not been met, and he simply needed confirmation, especially as his cousin Yeshua (Joshua aka Jesus) was also expecting the imminent divine intervention.
Another view would be that Luke was denigrating the party that was following The Baptist, therefore the authors of Luke (writing about 60 years after said event) created this passage in order to belittle The Baptist's followers.
We see that method employed by the writers of John's Gospel (20:24 ff) when they denigrated the followers of Thomas (John 20:24ff). The Gospel of Thomas had been circulating for decades before the John Gospel was created.
These writers were astute and highly political (in religious terms). Each party wrote in order to promote their own ideology and at the same time to argue against the views of others. Hence we are exposed only to the views of a writer and we have to infer what their opponents were saying. This is true, not only of the Canonised writings but also of later writers, such as the Church Fathers.
Doug