If I have my data stored locally
then it can only be stolen / lost through my own mistakes or incompetence.
However if it is stored in the Cloud then not only am I risking losing it through my own mistakes but also through those of other people.
It doesn't really matter how secure the online system is: If files are stored on an external device kept in my cupboard then not even the greatest hacker or most incompetent network admin on the planet can cause it to get taken.