This article doesn't seem to say why spanner won't do as well as it's clone.
"To put it bluntly, it would have been more interesting to see Google endorse CockroachDB and support it on Cloud Platform, creating an open source community as well as a cloud service for its Cloud Platform customers."
CockroachDB will presumably do better in the author's eyes because it's open source and there can be a real community built around it.