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As others have pointed out already, it's a combination of clearing the rights and financial incentives.
One thing to keep in mind is that as more people transition to ebooks, the incentives to release an ebook version will grow. Ebooks are still only 7-10% of the US market, and probably a tiny slice of the international market -- so there is 1/10th or less of the incentive to put any effort into the back catalog right now.
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