Because it is buggy (see the menu at the bottom of this screenshot) and inefficient in its use of space compared to Tapatalk (important when you're looking at a phone...good thing I have a big screen unlike an iPhone). It's also inefficient in general usage. For example, if I subscribe to a forum on the website, Tapatalk recognizes that and it shows up in my subscriptions. In Forum Runner, I have to go and add it to my favorites in the app as well (and vice versa). I also can't have the forum open to a user-specified area (it would be nice if it opened to my subscribed threads/forums).