Question

Why is expected income a large factor in choosing a retirement plan in which to invest?

Answer 1

Because you will be investing in it for the rest of your life. The yields should match the amount you are able to contribute. By doing this, you will be able the needs that needed to be made on current year while not throwing away the money for the future since the fruit of your retirement plan will be ripped when you're no longer in a productive age, 

Answer 2

When you are choosing a retirement plan to invest in, you need to figure out what the expected income from the investment is. When you start to invest, make sure you are aware of your current income and expenses and how those will go up or down once retirement hits. Making sure your end result covers your cost of living and expenses is the end goal, so the steps you take to acheive your goal is important.