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
Answer
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