This message was posted by Mod Poerkie a few days ago on Reddit and seems to be the reason why some devices are currently excluded:
Original message details are unavailable.
Hey all, quite a few people have mentioned that they are having issues getting into the RuneScape Mobile Early Access even though their device meets the minimum spec we’ve indicated. We are continuing to take steps to optimise gameplay on a number of devices however and as we do so, we have had to exclude them from access in the meantime. We will be working on more stability and efficiency updates and hope to open up Early Access to many of the devices that are currently blocked.
01-Nov-2019 20:27:44