Why was Bitcoin Core in need of a fork of LevelDB? Why did Bitcoin Core fork LevelDB? What special needs does Bitcoin Core have, which require a specialized version of LevelDB?
Sometimes LevelDB has bugs and those bugs sometimes are not fixed in a timely manner in the upstream LevelDB project. These bugs can be problematic for Bitcoin Core so a fork of LevelDB was created to handle these bugs specifically.

Additionally, to ensure security, we want to use a specific version of LevelDB at a specific commit. By having our own fork of it, we can control what version and what commit it is on so that we can control exactly what code we are putting into Bitcoin Core.