Here's the thing--The more you tax SSI, low-income earners, welfare recipients, etc., the more likely they are to need the very programs you're providing them. Or those on the fringes (not yet needing them) move into the needy category.
For example, when DH and I were newly married and VERY low income earners, had we been taxed at a higher rate, it's possible we could have moved (if we had no family support) into a category of employees needing govt assistance. By not taxing our meager income much, we were able to stay self sufficient. We didn't contribute a ton of tax $, but we also used no tax $ in the form of assistance.
So, tax those who can afford it and who won't come to need govt assistance by being taxed. Why tax those already needy or borderline needy?