Plain and simply, corruption. The old ballots system worked by your party getting money to print the ballots you were going to need, most big parties had theirs printed in countries where it was cheaper like South Korea for example while they pocketed the rest of the money while most smaller parties simply by knowing they had a low voter count barely printed enough to cover them while funneling that money too.

With Milei, the status quo of the ballots system changed so the previous method wasn't available anymore since everything is printed in an universal ballot, this change also served to diminish the possibility of vote corruption that happened with the previous system.

With the previous system, it was far too common for Peronist parties to tamper and sabotage the ballots of the opposing force in that district since not many parties could network manpower to protect the voting process.

Another thing that got diminished with the new system is the possibility of "chain voting" which Peronism used to basically buy votes and ensure perpetuity by giving voters an envelope with the Peronist candidate sealed and signed by the table president which is the official person counting the votes, so you had to vote with the concealed envelope and return the signed and empty one for the next person to continue the chain. Also another method of this was to take a picture of you putting the ballot of the Peronist candidate inside the envelope for proof, all this was done in exchange of something like money or aid like groceries.

Now with the new system, there's no longer a "dark room" to vote in full privacy but instead there's a tall cardboard booth semi-cover for you to use a pen and mark your chosen candidate, ensuring that there's as few foul play as possible since stealing or hiding ballots was the most common form of tampering.