As the above title states, I'd like some insight into how this decision has been justified.
What justifications have been used by politicians to enable people that are not mentally capable of making an informed decision to make that decision?
In a democracy, the (intended) point is that all citizens are represented. One's intellect should have no bearing on whether or not they are represented.
This is impossible to implement.
You would need to test and evaluate everyone to be able to decide who is "intellectually impaired".
There would be infinite battle between psychiatrist and other mental health specialists.
On what basis you would draw the line ?
Should we disallow "functionally illiterate" adult who cannot read or understand a political program ?
You open a can of worm that is impossible to close.
According to an article whose abstract is on the National Center for Biotechnology Information and which was originally published in Schizophrenia Bulletin under the title Schizophrenia: Nazi attempts to eradicate schizophrenia:
Although the Nazi genocide of Jews during World War II is well known, the concurrent Nazi genocide of psychiatric patients is much less widely known. An attempt was made to estimate the number of individuals with schizophrenia who were sterilized and murdered by the Nazis and to assess the effect on the subsequent prevalence and incidence of this disease. It is estimated that between 220 000 and 269 500 individuals with schizophrenia were sterilized or killed.
This total represents between 73% and 100% of all individuals with schizophrenia living in Germany between 1939 and 1945. Postwar studies of the prevalence of schizophrenia in Germany reported low rates, as expected. However, postwar rates of the incidence of schizophrenia in Germany were unexpectedly high. The Nazi genocide of psychiatric patients was the greatest criminal act in the history of psychiatry. It was also based on what are now known to be erroneous genetic theories and had no apparent long-term effect on the subsequent incidence of schizophrenia.
And see also the United States Holocaust Memorial
Wartime, Adolf Hitler suggested, "was the best time for the elimination of the incurably ill." Many Germans did not want to be reminded of individuals who did not measure up to their concept of a "master race." The physically and mentally handicapped were viewed as "useless" to society, a threat to Aryan genetic purity, and, ultimately, unworthy of life ... Despite public protests in 1941, the Nazi leadership continued this program in secret throughout the war. About 200,000 handicapped people were murdered between 1940 and 1945
Not only were they 'unworthy of life' but also unworthy of voting. Bringing these questions back onto the table, given the history associated with them, seems insensitive at best, and at worst - ominous.
(I realise it doesn't answer your question in the terms posited; but then again the question doesn't show much research or insight...)