Especially new assets and scripting that DLCs provide. You'd be a fool to try make something as expensive as this mod without utilizing every available resource.
Not really. Most large story DLCs for any Bethesda game require all expansions. I suspect it's for assets, but I would also just pick the largest use case (and already owning the expansions most certainly is the largest use case) and say it's a requirement also, so I'm not chasing down edge cases for people all day. Just the rough math of releasing something you have to support afterwards.