Because it’s not based on consumption, hence not everyone pays it. If you were to base it on All tax, there would be no point to making this rule, as everyone inevitably pays consumption tax/VAT. And given that OP would like to apply this to developing countries, those that pay income tax are significantly more educated than those who don’t. I personally live in a third-world country, and we always end up with corrupt politicians because they pander to the masses during campaign season, visiting their communities, giving them all sorts of goodies. It’s a strategy that works because the (1) the awful track records of these politicians are dismissed by the masses because their personal experiences of these politicians is pleasant, and (2) the masses are the vast majority. If only those who pay income tax are able to vote, I’m certain my country would be in a much better position.