Why do we perform Gunna only on Meem Mushada & Noon Mushada, is there any specific rule?

Question
Why do we perform Gunna only on Meem Mushada & Noon Mushada, is there any specific rule?

Answer
Assalaam alaikum.
The ghunnah is an intrinsic quality or characteristic of the noon and meem.  The ghunnah is a sound emitted from the nose, nasalaization.
You cannot say the noon and meem properly without the ghunnah; this is exemplified by someone who has a cold.  When they pronounce the noon or meem, the sound is imprisioned and does not sound normal.
The extended timing of the ghunnah exists for the noon and meem with a shaddah, but also with the rules of ikhfa' and iqlaab, but it is longer with the shaddah. 
The following links should give more information about the ghunnah:
http://www.abouttajweed.com/the-ghunnah-and-its-ranks/the-ghunnah.html
http://www.abouttajweed.com/the-ghunnah-and-its-ranks/ranks-of-the-ghunnah.html
http://www.abouttajweed.com/the-noon-saakinah-and-tanween/index.php
http://www.abouttajweed.com/the-meem-saakinah/index.php