Why is the case of genie called the forbidden experiment?

It's called the forbidden experiment, because to perform this experiment you would have to purposefully cause harm to the subject. To make a "feral child" the researcher would remove the child from parents, leave it in isolation, not care for it and mentally retard the child by making sure the it misses all the critical learning periods for language development. Not to mention the survival rates would be low.

Imagine a researcher wanting to cut chucks out of a child brain to see what happens?

That is practically what your doing in a manner of speaking.

Its cruel and inhumane, that's why its the forbidden experiment.