The drill down portion of the report is more of an interactive feature of reporting. The results depend on what row/cell you click on. So that is why that portion of the report cannot be sent out. It would would have to make some sort of assumption.
So if you already know you need a specific drill down to be sent out as well, you should schedule such a report separately.
You will need to create separate main (summary) and detail (list) reports and schedule each of them separately. You will need to do this for each summary type report that you are emailing.
The summary and drill down reports are defined separately as 2 distinct reports. The drill down report accepts parameters which allow it to display only the grouping that was selected by the user. However, even if the parameters could be passed to get the drill down report to include the complete details, the drill down report cannot be run as part of the scheduled main report task.