We accept a limitation on access to voting based on age. This has a hard, numerical boundary, and we can argue over what the boundary should be, but while a given age is in law it is simple and basically fair across all genders, races, cultures, languages, etc.

When you introduce a barrier like a literacy or knowledge test, you inherently restrict people from some or another culture, race, language, or whatever more than others. There is an inherent bias to how the test is written, presented, and judged. With all that wiggle room, people in power who want a certain outcome can see that the parameters of the test are arranged to fit their desires. There isn’t a version of this free from the potential for significant interference.

That, in a nutshell, is why there is always a strong argument against ANY such test as a requirement for voting. Some people may know better than others what policies will help people more, or at least think they do, but that doesn’t actually give them any more right than others in an equal democracy to make those choices.