Why does Allah (swt) tell the Prophet (saws) that when he comes across those who ridicule His revelations, he should turn away from them, whereas he has been tasked with conveyance?

The answer: Allah (swt) says in the verse that they (engage), this Arabic word means to dive, so it is as if they are diving in water. They are not speaking but diving, so the Prophet (saws) is to turn away from them until:

(… حَتَّىٰ يَخُوضُوا فِي حَدِيثٍ غَيْرِهِ… ) (…until they enter into another conversation. …): Why is this so? This is because, when a person enters into something which is outside of the limits of his knowledge and without competence, you will not benefit from discussing with him. Thus, you should turn away from him and leave him. And this is what exactly happened to the Prophet (saws).

(  وَإِمَّا يُنسِيَنَّكَ الشَّيْطَانُ فَلَا تَقْعُدْ بَعْدَ الذِّكْرَىٰ مَعَ الْقَوْمِ الظَّالِمِينَ… ) (…And if Satan should cause you to forget, then do not remain after the reminder with the wrongdoing people): This part of the verse addresses the followers of the Prophet (saws), but they are addressed through him.

Why is the forgetting attributed to Satan? The answer: This is because, the Holy Quran is the truth, and it is not possible for a person to forget the truth. However, the Satan is the one who makes people forget the truth.