Reasons I can think of:
Existing automation may be insufficiently good to cope with adverse weather and unplanned situations. Technology has moved on considerably since the Victoria, Central and Jubilee stock. We're automating cars now, after all. The article mentions a new signalling system being rolled out in support.
Lack of platform-edge doors. TfL have stated they won't run driverless trains without these. These are planned for installation on the Piccadilly line as part of the new stock rollout.