Because the Neo-Fidelius spell is highly unpredictable and no one knows what effect it would have.
There were three instances of its use in canon: Frank used it, lost his magic for a while, and it came back. George used it, lost his magic forever. Dumbledore used it, and it killed him.
For this reason the Neo-F was considered a brilliant effort, but ultimately a failed piece of spellcraft.
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Because the Neo-Fidelius spell is highly unpredictable and no one knows what effect it would have.
There were three instances of its use in canon: Frank used it, lost his magic for a while, and it came back. George used it, lost his magic forever. Dumbledore used it, and it killed him.
For this reason the Neo-F was considered a brilliant effort, but ultimately a failed piece of spellcraft.