Sam1911 got it pretty well.
While guns using such materials are often of low quality, the intent of the law has nothing to do with safety to the user or quality of the firearm.
All polymer frame firearms have frames with lower melting points, yet are widely used by police and military around the world. This clearly shows a quality firearm can be made on a frame with a low melting point.
At the time of such legislation polymer firearms were typically unheard of or not common.
It stems from a more politically correct way of outlawing "Saturday Night Specials" without using such language, and under the guise of safety.
The term Saturday Night Special stems from a racist term used to describe guns even poor blacks could afford. Poor urban members of society were involved in a lot of the violent crime, often on Friday and Saturday nights, the nights they went out partying on the town and getting into trouble.
The guns such poor people could afford or were found with were often the least expensive on the market. Whether they bought them, or stole them from a poor neighbor who could not afford much better.
Such guns became known as "Saturday Night Specials".
The term would be extended to poor people of all ethnic groups to describe the type of firearms they could afford.
Poorer urban neighborhoods typically had more crime, more drugs, people with less education, a rougher culture, and the least politically powerful demographic. (People living in such neighborhoods are also the most likely to need to defend themselves at some point.)
This led to gun laws that tried to disarm the poor, without disarming other members of society.
Because more politically powerful segments of society were less effected, they didn't fight such legislation to save the poor man's gun.
But they needed a way to legally identify the poor man's gun, even though everyone knew one when they saw it, that is not enough under the law.
They couldn't simply say "if it costs less than X it is illegal", even though that was the intent.
So they looked closely at the firearms to determine what differentiated them in a manner they could legally define. What they found was a frame more often made of materials that were cheaper to manufacture (just as low melting point polymer frames today are the least expensive to manufacture for similar reasons.) They realized these materials had a lower melting point than more expensive firearms, and so outlawed them on that basis.
This of course was nothing new. Similar knife, club, and other poor man weapon laws were passed in places decades prior when a knife was the poor man's weapon, and guns were possessed more by those with money to spare.
Just look at laws against things such as the Bowie knife in Texas, and various dirk and dagger laws other places, including places with great firearm freedoms at the time such laws were passed, meant to disarm the poor of their time.
