They were laughably easy to hack. If you go back to the early 2000’s, you can read up all about the Diebold voting machines and how vulnerable they were. Plus, it didn’t help perceptions that Diebold’s CEO was a major donator to GWB’s campaign:
Since then, many current Voting Machine companies have taken the advice of their detractors, and have worked to increase security on their devices while also providing a verifiable paper trail. (They also saw what happened to Diebold, who lost so many contracts due to the bad publicity that they had to let themselves be acquired in order to purge that name). The companies were basically shamed into doing a better job by all the voting security researchers (many of which are highlighted in that Wiki article).
The voting process is so much more secure and trustworthy than it was 20 years ago, mainly because of all the people paying attention to it.