Simply because Android is like the wild wild west of the phone world.
Lets narrow it down to say there are 4 different versions of android currently being supported and used. Then there's also about 6 generations of phone using those operating systems. So that's 6 x 4 = 24. Let's also say that for each of those phones there's certain region differences, hardward requirements (depending on goverment regulations and regionalized distribution). So lets say for every 24 phones there 10 different regional models. So that's 24 x 10 = 240. Now let's take all the different brands of phone into account there's about 20 "top" cell phone manufactuerers out there. So multiply that 240 x 20 = 4,800 different phones that the GoPro app has to be tuned to work on. Spread that across the 21 (or so) different camera models that GoPro has that support WiFi and that's 100,800 different setups that the GoPro app has to work on. This doesn't include tablets, non major brands, or custom made phones or android devices. It also doesn't include apps that people have on their phone that may interfere with the GoPro app, cause crashes etc.
So that's a heck of an undertaking that GoPro would have to do, a huge investment in time and people. So GoPro have to go with feedback from customers as to what phones they are using, which version of android they are using and prioritize the most popular phone brands, regions and models.
Once those phones are taken care of, then they move onto the next round of phones etc. So while youre phone may not be supported yet, that doesn't mean they aren't working on it.
When I first got my GoPro Fusion I was annoyed because it wasn't compatible with my Galaxy S3 tablet (that was over a year ago). With the brand new version of the app released a week or so back, now my Fusion works on the Galaxy S3. So yes, it took a while, but they got there in the end.
Hope this helps explain the complicated nature of getting the App to work on Android phones.
