You are so wrong. What was the "effective tax rate"? Bet it wasn't 70%. The left keeps quoting very high tax rates but doesn't tell us that these tax rates applied to extremely high incomes. They don't tell us of the tax deductions available to those in the 1950s. 

Theoretically the government should have collected a greater share of its income from wealthy individuals and corporations. So why didn't it?

Finally in the 1950s most of the world was recovering from www2 and much was in the hands of communist governments. Canada and the US had less competition from other countries. That's changed dramatically. I now wear shirts made in Vietnam, they didn't in the 1950s..