Thanks for this question. The goal of Blockcerts is to provide a common set of patterns so that credentials can be issued/verified across any blockchain, and across different market domains. When research started in 2015, the Bitcoin blockchain was really the only option.
In 2016, there was some discussion about spending developer resources to expand to Ethereum, and then the DAO happened. Ethereum’s hard fork and frequent exploits made it seem unreliable for credentials that need to last a lifetime. So, the people working on this project decided to make documentation for Bitcoin as useful as possible, while keeping it open to cover other blockchains in the future.
In 2017, Ethereum has gained huge momentum with developers and many are asking for Blockcerts to expand documentation (and reference implementations) to include Ethereum. Since this is an open-source community, I hope that developers will begin contributing to make this expansion happen, particularly since it will be pretty straightforward.
Further, as various networks begin utilizing the DID specification, it will become easier for Blockcerts verification to verify across any of these decentralized networks (Blockstack, Bitcoin, Ethereum, Sovrin, etc.).