There are scenarios where on Sitecore Experience editor or from Page's presentation details - Associated Content Popup, for particular rendering will see Create new Content is disabled.
To resolve above issue, I found that if we did setup Datasource template on rendering more than one. Then above feature will automatically disable.
Because if we have more than one type of template for data item to be selected on rendering. Then Sitecore will get confused that after click Create New Content, which type of data item will created.
Note: Please make sure only one type of template path set on Rendering's Datasource Template field.
