Because a Dark Ritual could force a sacrifice that adds redundancy. Leave the Basilisk alone, mostly satisfied.
Kill it and you are bound to transfer the knowledge to the next heir.
Attempt to kill the heir and end up as a horcrux.
Again, Harry wasn’t the Heir of Slytherin when Voldemort was plotting to kill him, or attempting to kill him, or even afterwards. He was only ever a Horcrux of the Heir of Slytherin.
Because a Dark Ritual could force a sacrifice that adds redundancy. Leave the Basilisk alone, mostly satisfied. Kill it and you are bound to transfer the knowledge to the next heir. Attempt to kill the heir and end up as a horcrux.
Again, Harry wasn’t the Heir of Slytherin when Voldemort was plotting to kill him, or attempting to kill him, or even afterwards. He was only ever a Horcrux of the Heir of Slytherin.