They have unmanageable security problems rather than manageable ones.
When I look at a ballot paper, I can see that it is what it is. When I stand in front of a voting machine, there is no was to validate that it's running the software that it's supposed to run. No way to validate it hasn't been tampered with, no way for the election scrutineers to validate it hasn't been tampered with before counting.
With a paper ballot, the scrutineers can lock the ballot box, and ensure it remains unopened and untampered with, aside from depositing ballots in it, simply by watching it.
