This is to maintain the probability, i.e. not all the people of the Scripture wish to misguide the believers. Only some of them do that.

(… لَوْ يُضِلُّونَكُمْ …) (…they could mislead you …): Why such a wish? Because the deviator despises themselves whenever they see the righteous because they couldn’t follow the fiducial path. Thus, they try to make the righteous deviate.