Isn't this because Wolf Lords aren't Chapter leaders, they're Company Leaders, and
WGBL aren't Captains, they are the leaders of the
WG.
But anyway, I think this discussion has been flogged to death.
Could you add
SW to the
SM codex? Yes, you could.
You would either have to remove a lot of the differences between
SM and
SW or write in a lot of special rules to capture the differences.
The first option isn't going to make
SW players happy, the 2nd option will result in a large, complicated and bloated codex (especially once you start adding all the special rules for
BA and
DA as well).
I, for one, don't want to buy an excessively complicated $150
SM codex to play one Chapter simply because the book also contains the rules to play other Chapters I don't play. I also don't want
BA/
SW/
DA trimmed to fit in to the
SM codex in such a way that doesn't cause bloat/expense.
I understand that other people might want to see a "big book of marine", or other people might want those other chapters cut back. This is where the "should they" rather than "could they" comes in to it and I really don't think we're going to see all that much agreement on it.