What social problem was the 16th amendment intended to reform?

The consolidation of economic power by the wealthiest. Prior to the 16th amendment, direct taxes had to apportioned to the states according to population. There were some exceptions based upon indirect taxes such as tariffs and excise taxes, but in general income from rentals of property were effectively unable to be taxed. The Pollock v. Farmers' Loan & Trust Co., case eased the governments ability to tax, in that it made wages exempt from the apportionment clause, but income via rentals, interest, and dividends were restricted to apportionment and effectively untaxable. The 16th amendment allowed for taxation on those sources of income.