See the New Democracy tag above, in particular the article on goals of voting systems. Plain paper works great in Canada, but is not so workable for the USA which may have 20 or more questions on a ballot. And the questions are different in each ward of each town. And the lobbies for the blind and disabled and non-English-speakers have a strong voice that opposes paper because they think machines which can assist disabled voters are a must.

The public of course would not care about open source, just the verifiers. But in fact, in the two-machine design with intermediate human readable paper ballot, you don't care all that much about the security of the first half so verification is nice but not essential.