One big barrier to online voting is that you don't want people to be able to be forced to show their votes which makes any kind of verification immensely difficult (you have to accomodate people needing to show a fake verification) because otherwise you leave things open to voter intimidation and vote buying.

Another is that by going online you instantly open the attack surface up massively. Currently to steal an election you have to physically move a large number of people to a broad set of voting sites or send in a lot of forged mail votes. Those both require physically doing something in the US where an online system lets anyone in the world potentiall attack the system.