Why did the scholars of Tajweed choose special rulings for the letters Noon and Meem, and not for other letters?

As for the emphasized (mushaddadah) Meem and Noon, they are by nature letters of ghunnah (nasalization), and their characteristic is not complete except with it. We have inherited the recitation of the Qur'an through continuous transmission (tawatur). As for the concealment (ikhfa') of the silent Noon and tanween, and the silent Meem with certain letters, the reason is that the articulation point (makhraj) of these letters is neither united with the articulation point of Meem and Noon, which would lead to their assimilation (idgham), nor is it far from them, which would lead to their clear pronunciation (izhar). The fundamental principle in recitation is through reception and oral transmission.