Hyperledger Composer supports the existing Hyperledger Fabric blockchain infrastructure and runtime, which supports pluggable blockchain consensus protocols.
But why Hyperledger Composer supports only Hyperledger Fabric and not others like Hyperledger Burrow or sawtooth?
Basically Hyperledger Composer is a wrapped version of Hyperledger Fabric. Composer is tool for FABRIC and it provides us with easy way of writing a business logics (aka Business network or smart contract).
It uses the backend Fabric to accomplish the features of the Hyperledger Fabric BlockChain Network like Consensus like Traceability, Finality, Provenance, etc.
Composer is aiming to develop its interoperability with other BlockChain Platforms in future.
Also I would compare Composer and Fabric with Wordpress and Core PHP respectively. Wordpress or anyother CMS written on top of PHP will only work with PHP on not with other technologies such as Ruby or Pearl.
Hope this clarifies.
As you correctly summize, Hyperledger Composer is pluggable and was designed to sit on top of different DLT technologies. In order for Hyperledger Composer to work with different DLT technologies assistance is needed from the community especially experts in these different DLT techologies to work with us and provide the appropriate plugins to work with other DLT technologies. Hyperledger Fabric was the first DLT chosen as this was the DLT we had expertise in.
