The effective income tax rate was still on the whole higher for the wealthy in the 20th century. Depending on where you put the percentile cutoff, I’m seeing peak-to-troughs between 5-20%.
And the reason most governments reduced the rate was because economists argued that the higher rate reduces economic activity so much that total tax revenue (collected by the government) is actually higher at the reduced rate. Look up the "Laffer curve".
What gives you that impression? I get that they are used that way at times, but the core purpose of taxation is to generate revenue for the government.