Using the grpc made sense with the earlier assumption that Libra could use the grpc serialization and sign transactions serialized with it. But when it turned out it actually cannot be used for this and Libra Canonical Serialization was developed, it was nigthmare to use the api: putting some information in structures generated from grpc protos and serializing some things with the LCS + similar work with de-serialization.
I think this new json-rpc api is also more developer friendly as you can create the api requests with tools like curl and postman. The integration is also more transparent when there isn’t the grpc library in between doing it’s magic and you can choose what ever http client available for your platform.
The only downside is the LCS serialization which needs to be implemented by the library doing the integration (at least if creating transactions is a requirement, all the queries can be done without it).