That's a great question. I don't know enough about them to answer, but I would like to hear other answers.

My hunch is Ethereum has more marketing behind it and maybe some more well-known investors or industry people promoting it. It seems the BitShares/Steemit ecosystems are built by developers for developers without much thought beyond just the actual technology. It's all quite fascinating to me how the technology itself isn't what really matters when it comes down to which one gets more marketshare.