Why not return of income averaging

With all of the tax cuts for the ultra-rich, it would be nice to see a return of one that Reagan did away with, that can be really helpful to the middle-class : income averaging. Income averaging taxes on the average for a certain period of years, eg., 5 years. So if your income greatly increases or decreases, you have the option of calculating your average income for the last five years, and recalculating the taxes that would have been owed. If you would pay less tax by income averaging, you have the option to choose to do so.
For example, if you earned $10,000 for four years, then earned $35,000 , the average income for each year would be $15,000.
(These hypothetical tax rates are made up on the spur of the moment; they are only to show the mechanics of the calculations).
If the tax rate on $10,000 is zero, and the tax rate on $35,000 is 15%, the total taxes for the five years without income averaging would be 0.15*35,000 = $5250.
If the tax rate on $15,000 is 5% (which is 0.05), then with income averaging, the taxes for the five years would be 0.05*15,000*5 = $3750. So there would be a savings of $5250 - 3750 = $1500. In this example, if the tax rate on $15,000 was 7% or greater, you would be better off not doing income averaging.