Rubber membrane keyboards are easy and cheap to manufacture. It is one large membrane for the entire keyboard and when the rubber dome is pressed down all the way by the key you press, that key is registered by the computer.
Mechanical keyboards are more expensive to manufacture than membrane keyboards. Each key will have it's own mechanical switch to register when the is press. The more keys on the keyboard, the more switches it will have. Also it is not necessary for the key to be pressed all the way down to be registered; about have way is enough. This makes it faster to type on a mechanical keyboard than a membrane keyboard.
Most high end mechanical keyboard has a feature called "n-key rollover" where the keyboard itself scan each key individually and can allow all the keys to be registered by the computer if you were press all or as many keys as you can simultaneously. This feature is only supported by the PS/2 connection, not the USB connection. For Razor's BlackWidow line of mechanical keyboards only have "n-key rollover" for the A,D,S,W keys.
Membrane keyboards only allows for at most 6 keys to be registered when pressing them down simultaneously. Attempting to press more than 6 keys down simultaneously can result in no keys are registered to at most 6 keys.
Lastly, if the keyboard's membrane develops problems such as not registering a specific, then the entire keyboard must be replaced. Since a mechanical keyboard has a switch or each key, it is a simple matter of replacing the switch.