It's a lot easier to hijack online voting infrastructure than it is to start digging through everyone's mailboxes at the correct time when they happen to actually be sending a ballot out.

The voting period in Washington is 18 days. So someone would have to either be constantly looking through my mail for the better part of three weeks, or have the best luck when it comes to predicting when I would actually send it out, and that's if they got the correct mailbox and I didn't, say, go to a post office or the ballot dropoff boxes. It would be hard to do even for an individual, and much harder to scale up.