The question is, what's the alternative?
Suppose you roll out a new voting schedule expanding voting hours. The purpose is to make things more convenient, not affect election outcomes -- someone who might have had to miss an hour of work to vote can now do it during off hours instead etc. But without having any way to know this ahead of time, the new schedule turns out to change who votes in a way that disproportionately benefits one party over the other. What should be done?
There is no logical reason for one schedule to be more fair than another, assuming that everyone has a reasonable opportunity to vote in both cases and the difference is only a matter of convenience. The problem is that convenience can affect election outcomes -- and there are arbitrarily many possible schedules.
Suppose that increasing voting hours at certain other times (e.g. opening polls earlier in the morning) would benefit Republicans more than Democrats. If they did that instead of removing the hours that disproportionately benefited Democrats, you could accuse them of the same thing -- changing the schedule to affect the outcome. But they're also just restoring the original balance that existed before a change was made that wasn't expected to alter it but did. On the other hand, who is to say that the original status quo is more fair than some other thing?
And that's the problem. Nobody has any more right to slightly more convenient voting hours than anyone else, but who gets them can determine the election. The incumbents are the ones who get to make the decision, so what would you expect them to do? More importantly, what principled thing do you propose to do instead?
In theory they could allow voting 24 hours a day for a month, but the expense of that likely exceeds the marginal benefit. Meanwhile as soon as you start deciding which hours to cut, you're potentially choosing who wins the election again.