I left SS income off my 2018 return. why was something like this not picked up by my purchase and use of the Premier package?

Income can come from thousands of sources.    TurboTax does not know the source of your income unless you tell it.     If you went through the entire interview without bypassing sections then it would ask about SS income in the retirement section.    The bottom line is that you are responsible for double checking your return before filing to be sure that you reported all of your income.

simple you either bypassed the question or when asked click to continue or answered no