Please explain the reasons why Izhaar if noon Saakin is done in the huroof Ul halq....

Question
 Please explain the reasons why Izhaar if noon Saakin is done in the huroof Ul halqi, Idghaam is done in the letters of Yarmaloon.
Answer
We don't normally need to find a reason for the tajweed rules, as they correspond for the most part to the natural tongue of the Arabs at the time of revelation and the Qur'an was revealed with tajweed. 
In the case of the ith-haar of  the noon saakinah when followed by one of the six letters pronounced from the throat, the scholars have pointed out that the reason is because of the great distance between the articulation points, where the noon saakinah is pronounced from the tip of the tongue and the gums of the top two front incisors and the throat is very far from that. 

The idghaam of the noon saakinah into the letters of the group "yarmiloon" is due to the closeness in articulation points and in some of the characteristics of the noon and the letters yarmaloo (ya', ra', meem, laam, wow).  The noon of course has the same articulation point and characteristics as a noon, since they are the same letter and that is the reason for the idhgaam .