I don't understand why Mommie Dearest was made.

The producers, having purchased the rights to a best selling book, hoped that lightning would strike twice and that the film would attract large audiences, and therefore that they would make money. It didn't (at the time anyway) and they didn't.

Joan wasn't around to sue anyone and the nearest relative was the one that wrote the book....

Whatever one thinks of the film Dunaway is aces as Joanie. An imitation silk purse from a sow's ear.