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To the technical reasons of having another blockchain intergrated with your system im not sure how STEEM comapres to others which I think could be one issue. But i'm pretty sure coinbase wants to avoid currencies that would die and be delisted they're very picky with what they list unlike say a kucoin or cryptopia.

They want currencies with real volume so they can A attract more users and B have more trade volumes and valuable trading pairs on their platform

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