Scantron doesn't allow for write-in candidates and it's very easy to design an unusable scantron form that's confusing and difficult to follow. Another issue is that often the scantron circles are small and too close together, making it difficult for people without fine motor control to vote: the solution is a form with much larger circles and spacing but that would be wasteful and inefficient for the majority of voters.
The Washington voting system allows for write-in candidates despite also having a bubble that the voter fills in with ink or pencil. If the write-in votes outnumber any of the winning candidates, they could hypothetically count them by hand, but that never really happens.
You fill in the center of the arrow for who you're voting for, and there's an entry at the bottom for writen-ins. Afterwards this is scanned by a machine and the paper put in storage.
