I'm not sure why you'd need a third party to work with Google Play Books? If you have a PDF or eBook, you just upload the file to their backend just like Itchio. (Also, yikes! Draft2digital takes 10% of every sale on top of the other site fees?! That's INSANE. Is it really so convenient that it's worth that much??)
Ooh, I see. If I had to guess, it's because folks who are into indie trans stuff seem to be pretty attracted to the idea of Itchio: it (had) good tools for discovery work like yours and covered more than just novels. Before the paypro stuff, it also started getting a reputation as the "hip new place" to buy books, so I'm wondering if it's that. (Plus, I've found that folks usually just like a place that's easiest to buy from and Itchio up until recently made it pretty painless.)