Ahh, I thought you were referring to their quality rather than popularity. Even so, while I will grant you Up, Up, and Away isn't very well circulated, Last Son and Brainiac are. Both are still available to purchase in one trade collection: https://www.amazon.com/Superman-Last...s%2C157&sr=8-1
And Brainiac was adapted into a standalone animated movie. Plus, the DC App highlights both stories as major Superman stories on the comics section of their app. Both are very easy to find. So I wouldn't call either of them forgotten.