One reason could be that the author lost interest. Maybe they moved on to other fandoms or just got tired of writing about Petrie. Another reason might be that they faced some real - life issues that made it impossible for them to continue, like getting too busy with work or school.
Well, for 'Land Before Time Petrie' fanfiction, sometimes the story just doesn't develop as the author initially planned. They might start with a great idea for Petrie's adventure, but as they write, they realize that the plot isn't working out or that they've written themselves into a corner. Additionally, changes in the official 'Land Before Time' canon can also make it difficult for fanfiction authors to continue. For example, if new information about Petrie or his species comes out, it might conflict with what the author had already written, leading to abandonment.
I'm not sure where to specifically find abandoned 'Land Before Time Petrie' fanfiction. You could try searching on some old - fashioned fanfiction archives or forums that might have such works stored away, but it's a bit of a long shot.
One possible fanfiction could be about Ducky and Petrie going on a secret adventure. They might discover a hidden part of the valley that no one else knows about. Maybe they find a strange new type of plant or a small, friendly creature that they befriend and have to protect from the larger, scarier dinosaurs in the area.
One common theme could be loneliness. Since Petrie is a bit different from the other characters, he might be left out or feel alone, which can be a sad situation in fanfictions. Another theme could be fear. For example, he might be afraid of not being able to keep up with his friends during a dangerous journey.
A plot could be that Ducky and Petrie find a magic stone that gives them special powers. Ducky can now speak to all animals, not just dinosaurs, and Petrie can fly super fast. They use these powers to help their friends when a group of mean dinosaurs tries to take over their territory.
I'm not sure of specific 'Land Before Time' Petrie sad fanfictions off the top of my head. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Land Before Time Petrie sad' in their search bars and you might find some interesting ones.
One possible reason could be jealousy. Maybe his sister is jealous of Percy's special abilities or the attention he gets. Another reason might be that she is forced to abandon him due to some external threat or danger that she believes will harm her if she stays with Percy. Or perhaps she has been brainwashed into thinking that Percy is a bad influence or a danger to her.
It could also be that Harry himself is so traumatized by the war that he isolates himself. He might push people away, thinking that he's a danger to them. As he withdraws, others might misinterpret it as him not wanting their company and gradually abandon him.
It could be that there was a prophecy related to his sister that made the family think she was more important. For example, a prophecy that she would be the key to saving the village in the future. And in their short - sighted view, they abandoned Naruto to focus all their resources on her.
One reason could be that the author lost interest in the story or the character. Maybe they started another project that they found more exciting.
Sometimes, the source material itself can be a factor. If there are major changes in the official My Hero Academia storyline related to Izuku, it might make the author's original concept for the fanfiction no longer viable. Or perhaps they had planned the fanfiction to be part of a certain arc in the series, and when that arc didn't play out as they expected in the official story, they abandoned their work.