I think the rationale is that the existing incidence of local ballot fraud points at a widespread, under-reported problem that doesn't inspire confidence in the process.
Voting by mail is pretty normal to me, being in the military and outside of my home state for most of my adult life, but it's about as secure as WEP encryption for your Wi-Fi.
https://www.inquirer.com/news/voter-fraud-philadelphia-ward-...
https://www.dailybreeze.com/2016/11/03/how-more-than-80-elec...
https://www.heritage.org/election-integrity/commentary/datab...