We discussed this a little bit in the thread about the film trailer. For the film specifically they were trying to cram in as many signature monsters as possible, and monstrous shape is something Druids could do in older editions. Still holding out that it'll be something added back to high-level Druids in the future, but moon Druids are already overtuned, so it would need some thought behind it other than just "owlbear cool".
