It's always insidiously attractive to deny voting rights to groups of people who, in our own opinion, do not have good character or good judgement. It certainly
sounds clever to say we're implementing rules so only the best and the brightest can vote which (we argue) will lead to better decisions and a higher caliber of elected leaders. And in fact such arguments often
have carried the day and lead directly to such policies being implemented. In various places and at various times, women, ethnic and religious minorities, felons, unmarried males, landless people, people who could not pass a reading test, and many other small powerless groups have all been denied the right to vote. However, the result has never been "smarter" political decisions and "better" governance, but only ever the political oppression of the disenfranchised group.
It makes more sense to view voting rights as political capital that forces politicians to care about the needs of particular voting blocks then as a ticket to participate in some impartial decision process. The implication of this point of view is this: to get a more equal society, or at least to prevent certain extreme example of inequality, we need to make sure that as many people as possible have the right to vote.
The process of deciding who can and cannot vote is also fraught with moral hazard and the potential for corruption. A literacy test before voting sounds reasonable (if you believe we only want smart educated people voting) but it leads to corrupt situations like this:
https://slate.com/human-interest/2013/06/voting-rights-and-t...
This is a rare example of a real slippery slope: once you start the process of identifying small "undesirable" groups to disenfranchise, the process is unlikely to stop in anything short of full blown dictatorship.
https://en.wikipedia.org/wiki/Nuremberg_Laws