It's been brought up before and there is almost no way you could do everything in the game with it. The combat isn't what it once was, you're not just clicking on enemies. The #1 reason it wont happen officially is because the people on different consoles wouldn't be allowed to play with each other and Jagex would rather just not release it in that case.
It would require a touchscreen specific UI to do anything but skill competently and Jagex aren't anywhere near devoting that much time to it. At the moment, if it works on other devices great, if not, they are 100% focused on the PC client. Maybe later, but it would still be a lesser control scheme.