I don't know much of anything about how these things work, but wasn't the original contract all tied in with New Line Cinema and however that corporation linked to the Tolkien estate? If so, there'd probably need to be a new contract to enable new models for the new series to happen.
Also, everyone knew the movies would be massive, but there's less certainty over these 'spin-off' series and there are huge implications for SBG including possibly brand new (or at least 'revised') factions so it would be a huge undertaking to revise the rules, lists and model ranges to take in a series that may or may not be successful enough long term.