Why can't the schema owner create a new table?
If you are using SQL Server 2005 or later then to create an table in a schema you need both CREATE TABLE at the database level and ALTER at the schema level. Ownership of a schema covers the ALTER permission requirement but not the CREATE TABLE one.

CREATE TABLE is also granted by membership in the db_ddladmin fixed database role. This role is for backwards compatibility - do not use it for new security implementations.