Some might write it for shock value. They want to create something that stands out and gets a strong reaction from readers. It could also be a way to explore very extreme and unconventional character relationships, pushing the boundaries of what's possible in the Harry Potter universe.
Some people might write it for shock value. They want to create something that is so out - of - the - box and unexpected that it grabs people's attention. It could also be a way for them to explore very extreme and dark 'what - if' scenarios in the Harry Potter universe, even if it goes against the original story's essence.
It's not a common trope in the general Harry Potter fanfiction community. Most fanfiction tends to focus on more traditional relationships and storylines within the wizarding world. This kind of plot is rather outlandish and goes against the typical character dynamics and the overall tone of the original series.
In some fanfiction, Harry leaves to seek more powerful magic. He realizes that the magic in Britain is somewhat limited in the fight against Voldemort. So he travels to other places, like ancient magic - rich regions in Asia or Africa, to find a way to defeat Voldemort. He hopes that by leaving, he can gain the upper hand in the battle.
One reason could be a prophecy misinterpretation. Voldemort might initially think that Harry is key to his success in a different way than he originally thought, and so he saves him to figure out this new angle. For example, if there was a prophecy about a 'chosen one' and Voldemort realizes he needs Harry alive to fulfill some part of it that benefits him.
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.
It could be that Harry has lost all trust in the Order of the Phoenix or Dumbledore for some reason. He might think that Voldemort, being the most powerful dark wizard, has the means to protect him from whatever threats he is facing. Also, there could be some false information that leads Harry to believe that Voldemort has a change of heart and can be trusted to protect him.
One reason could be for comical effect. It's quite funny to think of the powerful and feared Voldemort as a pet. Another reason might be to explore a different power dynamic. If Voldemort is a pet, Harry has a certain level of control over him, which is a complete reversal of their normal relationship in the original story.
Perhaps Voldemort discovers that there is a greater threat out there. In the fanfic, it could be that another dark force is emerging, and Voldemort sees Harry as an ally of sorts against this new threat. So, he decides to protect Harry in order to deal with this common enemy.
It's against the family - friendly nature of Pokémon. Pokémon is aimed at a wide range of audiences, including children, and such a plot would be inappropriate.
One possible reason could be that he is wrongly accused of using dark magic. In some fanfictions, the Ministry of Magic might be more corrupt or easily influenced, and they could frame Harry for something he didn't do, leading to his expulsion.