You sort of have to look at the history of AFF and apps to fully understand, but years ago there was an app when we used vBulletin. It was a skinned version of one provided by Tapatalk. It worked quite well at the time, some interesting features, but Tapatalk got too big for their boots and their business plan changed - they would have run their ads in the app to make money. This was the beginning of the end. After that an app was developed by an independent developer, but it was never polished enough. With the change to Xenforo and a much more modern interface that allowed for user interface elements to be dynamically sized on devices, AFF discontinued the app. There was some criticism at the time, but eventually users worked out that an app wouldn't do anything a browser or web app couldn't already do. The next point release of Xenforo is just around the corner, who knows how that may revamp things yet again.
Disclaimer: Sometimes I might say we when I mean AFF, as I used to work here but I am not part of the we anymore 
