I figured this out. The sheets tabs actually included in the toolbar. As we set toolbar only to specific .qvw file in the qlikview server, that is why it "disappear" in the new .qvw file that I just added.
You can find the way to make the toolbar available by modifying the opendoc.htm file here.
After modifying the opendoc.htm file, add it for specific .qvw file as below: