One possible reason could be as a form of humiliation. His enemies might take his glasses to make him feel vulnerable and less in control. Without his glasses, Harry is severely visually impaired, so it's a way to undermine his confidence.
It could be part of a magical curse. The curse made him lose his glasses and trapped him in a cage. Maybe it was cast by a jealous student at Hogwarts who wanted to get back at Harry for always being in the spotlight. In the cage, Harry has to figure out how to break the curse and get his glasses back, all while dealing with the limitations of his impaired vision.
Another reason might be related to a new magical artifact or potion. Harry might have come across something that had an unforeseen effect on his glasses. It could be that a new charmed item he was using interacted with his glasses and caused them to disappear.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
There are likely some on Archive of Our Own (AO3) as well. Look for tags such as 'Harry Potter' and 'without glasses'. You might find a story where Harry is separated from his glasses during a Quidditch match and has to play without them, which could lead to some interesting plot twists and character development.
A possible plotline could be that during an eclipse, Harry's glasses become a portal to another dimension within the wizarding world. He steps through and finds himself in a place where the rules of magic are different. Another plotline might involve the glasses being enchanted by an eclipse - influenced spell that causes Harry to see things that others can't, like hidden messages on Hogwarts' walls.
One reason could be differences in strategy. Harry may have a more direct or risky approach to fighting Voldemort, while the Order might prefer a more cautious, long - term plan. This clash in ideas can lead to his anger.
One reason could be a desire for revenge. Maybe in the fanfiction, Voldemort has done something even more heinous to Harry's family or friends, driving Harry to take extreme measures.