The mouse idea hasn't changed because it works. It's efficient enough and there aren't really any problems to point out about it. The mouse has all the neccessary functions for operating a computer. The mouse itself is easy to use, you dont' need much knowledge to know how to use it, you can buy it anywhere, its not that expensive to buy, its fairly durable, and its basically universal to any computer out there.
Another reason why the mouse idea hasnt' changed is because of compatibility. All programs out there are most likely adapted to the mouse. If you were to invent a new piece of hardware controller, you'd have to create some drivers etc...from which all programs would be able to recognize etc...
Its like the old battle between Mac and Windows, applications built for windows will only run in windows, and vice versa. (the time before they managed to fuse their programs together with compatibility features)
Hope that helps yah out