I am not very sure, but seems in VoltDB, there is no such concept like "Schema" in MySQL, which is a logical database instance, so can group logically related tables and stored procedures etc together.
Any idea why it is so?
VoltDB does not support multiple named schemas, only one unnamed schema.
There is no architectural reason why such a feature couldn't be added in the future, but it hasn't been frequently requested. Typically separate applications are deployed on separate clusters as they have different needs, both initially and as they scale.