The tale of Cinderella has changed a lot over the past few centuaries:
Finally, the glass slipper is peculiar to Perrault's telling of the story, which is one of the world's best-known and most widely distributed folktales. In most versions, Cinderella is helped by her dead mother, who reappears as a domestic animal, typically a cow or goat, rather than her fairy godmother; often, she makes three visits to a ball, festival, or church; and her true identity is revealed by a ring that will not fit anyone's finger but hers. The story probably is of Oriental origin. In the oldest known version, from China in the ninth century, the heroine loses a slipper, as it happens, but it is of gold. The glass slipper, then, along with the use of the witching hour of midnight as the moment at which the heroine's finery will disappear, seems to be one of Perrault's own contributions to the Cinderella story.
And on wikipedia:
They tell the fabulous story that, when she was bathing, an eagle snatched one of her sandals from her maid and carried it to Memphis. While the king was administering justice in the open air, the eagle, when it arrived above his head, flung the sandal into his lap. The king, having been stirred both by the beautiful shape of the sandal and by the strangeness of the occurrence, sent men in all directions into the country in quest of the woman who wore the sandal. When she was found in the city of Naucratis, she was brought up to Memphis and became the wife of the king
In 1893, Marian Roalfe Cox, commissioned by the Folklore Society of Britain, produced Cinderella: Three Hundred and Forty-Five Variants of Cinderella, Catskin and, Cap o'Rushes, Abstracted and Tabulated with a Discussion of Medieval Analogues and Notes.
It seems though that the first use of glass slippers is by Perrault in 1697:
One of the most popular versions of Cinderella was written by Charles Perrault in 1697. The popularity of his tale was due to his additions to the story, including the pumpkin, the fairy-godmother and the introduction of glass slippers
As Snopes will tell you, he artificially (as opposed to through mistranslation) added the glass slippers. Before then other items were lost, stolen or left behind. Sometimes nothing was left behind.
So in-universe there is, by and large no glass slipper being left behind. And therefore no reason for it return to glass.
If you are referring solely to Disney/Perrault, there is a subtle hint that the fairy godmother kept the slippers glass so the prince and Cinderella could be reunited:
He had Cinderella sit down, and, putting the slipper to her foot, he found that it went on very easily, fitting her as if it had been made of wax. Her two sisters were greatly astonished, but then even more so, when Cinderella pulled out of her pocket the other slipper, and put it on her other foot. Then in came her godmother and touched her wand to Cinderella's clothes, making them richer and more magnificent than any of those she had worn before.
If the godmother was already hanging around, and just transforms her clothes, one could argue the return to mundanity by midnight was an artificial limit placed on Cinderella by the godmother. There was no inherent weakness of her magic, and thus she could have just excluded the slippers for the sake of hooking Cinderella up with the prince.