With the rule idhgam, why don't the lettesr waw and yaa have a shadda

Salam aleikum,

My question is: With the rule idhgam: why don't the letters waw and yaa  have a shadda on them whereas raa, meem, lam, noon have a shadda on them?
BarakAllah feekum

Wa alaikum assalaam wa rahmtaullahi wa barakaatuh,
The reason there is no shaddah written on the  and   when the  saakinah or tanween merge into them, is that the idhgaam is incomplete ( ) in that the ghunnah of the noon remains.  For that reason most copies of the Qur'an do not have the extra marking of a shaddah. 
Please note, the shaddah is not part of the 'Uthmani writing but part of the enhancements done later in time by scholars to aid in the correct reading of the Qur'an. 
Please see: http://www.abouttajweed.com/the-noon-saakinah-and-tanween/merging-of-the-noon-saakinah-and-tanween-p.html

Wa feekum al-barakah