Beyond the usual reasons there are others. In a straight up/down voluntary voting system a politician has extra reason to invest 'get out the vote' efforts in places where they have maximum effect... cities and college campuses, liberal territory basically. You could do straight up/down in a mandatory system but then you're deliberately including votes by people who are too lazy to go through the minimal effort to vote and thus probably are too lazy to be informed on issues and aren't really ideal voters.

In the US, older people have more effective say as a group since many more of them vote, and we like that as older people generally are better informed, your average 55 year old knowing more then an 18 year old. In a mandatory voting system there will be more 18 year olds then 55 year olds voting, as population grows, and that's not really a bias one might consider ideal. Absent that, in a voluntary system, it is simply far easier to grab those who would be unlikely to vote and get them to vote in a city or near a college then in the country, so you'd be creating a doubly non-ideal shift to the vote.

Currently, say, if the Governor of New York created a tax break or cash reward on the spot for voting, and pulled off say 80-90% vote totals, while the end sum might be a decent amount different then prior breakdowns, it wouldn't infect the national vote, NY has NY's votes based on their population nothing more. A 40% turnout in Indiana and a 70% turnout in Arizona, both population 6.5 million, will get the same result, even though one had 2.6 and the other 4.6 million voters. So the current system eliminates the advantage to any heavily republican or democrat state... the sort who would have a one-party state gov't... from having any reason to get their hands into the pot, those states have no motive to tamper with the EV as it is already a foregone conclusion, and those states are also the ones that could most easily pull off some sort of legitimate or fraudulent mass vote effort.

The billion or two spent on POTUS races is less than a percent of a percent of the economy, but effects far more, and a populace state with a candidate up for POTUS could easily find some way, in a pop vote system, to seriously spike the odds, even if simply by that state voting more on account of that person coming from there, so we might end up never seeing a candidate who didn't come from the top 5 most populous states. This is not to say our current system is wonderfully fair, but devil you know, and I don't think most pop vote fans or fence sitters have ever considered much of what I listed there.