Android is an extremely diverse platform that manifests itself in a simply massive volume of forms across a vast variety of different hardware, and that's the biggest problem that Epic Games has run up against in getting Fortnite Battle Royale out to the wider Android world. The team has had to face a long list of difficulties stemming from forcing a console game to run on disparate hardware, especially when it comes to older and weaker phones. Even among modern devices, there are outliers that do one thing or another differently at the system level and make it difficult or even impossible to use the same implementation of certain game conventions across the board.
One of the biggest roadblocks tied to Android fragmentation has been memory usage. Many phones will destroy apps when they go past a certain memory usage threshold, or will compound memory problems by killing an unused service in the background to free up memory, then restarting it after a bigger app has taken up the memory it was asking for. This can result in all sorts of unpredictable issues. The game was given four graphical presets ranging from Low to Epic, which helped the issue in some ways by turning off certain options, moving the view distance around, and otherwise making the game easier on some devices. The Fortnite team, working with experts from Google, managed to figure out that emulating a core graphical buffer functionality allowed the function to run as it was on a larger variety of devices, and saved a bunch of memory in the process. On average, using this new approach shaved 400MB off of the memory usage in a typical game, which can be a lot when devices like the Google Pixel 2 only let apps take up to 1.8GB of RAM. Implementing the somewhat new Vulkan graphics API also helped with memory usage and CPU cycles when it came to draw calls, but this new API and its 20% CPU cycle savings, is only available on the Samsung Galaxy Note 9 and Galaxy S9+ for now, and it required Samsung's help.
What this all boils down to is, quite simply, every Android device is a little bit different. This means that Epic has to tweak the game to accommodate each device on an individual level, generally speaking, and this process takes time and work. Unfortunately, this means that many people, even those with fairly modern devices, are left to wait for as long as it takes to optimize the game to run satisfactorily on their devices. The Fortnite team cautioned in their blog post that one release they put out resulted in many players having trouble with crashes and other misbehavior, so even once the game is released to your device, keep in mind that porting it over is a complex process and it may not be entirely playable at first.