I've received a few questions about why my books aren't all available on Smashwords/Nook/Apple etc. Most of these decisions are ruled by the cold, hard realities of the business and a desire to keep them available to as many people as possible. Everything that is eligible to appear on Smashwords is sold through Smashwords, but as soon as I put a bundle into Amazon's KDP unlimited program, their terms and conditions mean all its constituent stories must be removed from other sales channels. I try to have all books be available on Smashwords for at least a year before I lock them into KDP. I'd like to keep them up longer, but my readership through KDP dwarfs that of any non-Amazon channel. A lot of people only read the bundles.
Apple/Nook have their own set of issues. These days, their content guidelines are very strict. Many of my earlier books are prohibited. They're also a tiny market compared to both Amazon and Smashwords, so when I'm pressed for time, they tend to be what gets neglected. I'm trying to get them up to date, but my tech catastrophe a few weeks ago is causing problems...
There are actually two stories currently available on Smashwords only! Her Best Friend's Futa Flight and Her Stepmother's Futa Heirloom