1. Bulk mail is slow. First class mail takes a few days to be delivered. Bulk mail may take a couple weeks.
2. Bulk mail must be sorted and grouped. Depending on volume, you're sorting by either zip or zip+4. This is easy to do when you can just sort your mail merge file so that you print in the sorted order. For presorted mail, sorting machines aren't needed.
Basically, completed ballots are going to be flowing in the direction the mail system isn't optimized for. I'm sure they'll all get there eventually, but ensuring they all arrive at the right place in time to be counted is the hard part.