I mean the dev is obviously free to work on whatever he wants, but I've had a Tildes account for like 2 years now, and honestly it never got any traction. Lemmy on the other hand is getting a lot of users, and it is desperately missing a non-buggy app.
I'd already been working on a Tildes app for a while before knowing Reddit was going to kill third-party apps. I didn't have time to work on Lemmy and Tildes and RIF.
Since Tildes is different enough, it made sense at the time to have both a Tildes and RIF app which didn't have too much overlap; their communities are quite different. Lemmy and RIF are more similar to each other.
Who knows; after I release my Tildes app, I could work on other apps too.