Why is Qalqala classed under Ghair Mutadada?

Question
Asalamualilkum Warahmatullahi Wabarakatuhu. Why is Qalqala classed under Ghair Mutadada? Couldn't there be an opposite to it i.e. not having the echoing sound when the letter has a saakin? Jazakallah Khair.

Answer

Wa alaikum assalaam wa rahmatullahi wa barakaatuh
The early scholars of the science of tajweed put together the characteristics of the letters in to two groups: the characteristics which have opposites, in which there is one characteristics and and opposite and every letter is one or the other, and the second group of characteristics which have no opposites, meaning only a few letters or only one have a particular characteristic, with no opposing characteristic.
For example with the characteristic -which is running of breath and its opposite which is imprisonment of the running of breath due to relying on the articulation point.  Every letter in the Arabic alphabet has either running or imprisonment of the breath (air), and both of these characteristics are found and apparent in proper recitation of the letters. 

In the category of characteristics without opposites, it is a specific characteristic found in a few or only letter and there is no opposing characteristic to be described or heard.  It is either a particular characteristic preset or not present., such as -spreading the sound only found in the letter
In the case of the qalqalah, not only is it only present with a small group of letters with no opposing characteristic, it is only found in the state of the particular letters having no vowel.  The mistake of making a qalqalah on a letter outside of the group of qalqalah letters is meaning you are applying a characteristic not found in the letter. 
Wa iyyaakum