This is fairly obvious; the simpler you make something, the easier it will be to use. If you think about a car remote locking control, it only has one button – to turn on and turn off the locking system.
Simple = easy to use.
Now picture one of those programmable universal TV/Hi-Fi/DVD/etc. remote controls – take the best one you can find, the one that controls the most equipment. Now explain to someone else how to set it up.
Flexible = difficult to use.