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To leverage the brand. They are still interested in the brand and the RPG is helping keep the brand alive.
The only problem I have with that theory is this: if there isn't enough money for Hasbro to be interested in the line, why spend two years (and goodness knows how much money) on developing a whole new edition?
Though I suppose it's just about possible Mearls (or someone else) could have persuaded them it's worth it for one last, big payday.
('Big' relative to D&D sales, obviously.)
To leverage the brand. They are still interested in the brand and the RPG is helping keep the brand alive.