Because people buy her singles based on the preview, end up not liking them because they're over generic and then only 1/8 of them end up buying the album. Simple as that
It's interesting to see that every artist when was at their peak was able to sell albums, except for Katy: Gaga and her monstrous sales during TFM era, Britney (
) and BOMT and OIDIA, Adele, Avril etc... When you look at Katy, you only see strong single sales and lackluster album sales in comparison.