Interestingly, I heard a podcast or read a book not long ago (I'd guess John McWhorter, but don't quote me on that) that touched on this, and the author attributed this simplification from the Indo-European norm of heavy grammatical gender to the fact that both Persian and English achieved their modern forms largely as languages learned by adults - the Vikings mixing with Saxons, the various subjects of the Persian Empire - and thus they tended to simplify endings.