The main reason for this is simply because of the way commercial printers work – they work on a colour model called CYMK (also known as Cyan, Yellow, Magenta & Key – Black).
Our monitors work on a colour model called RG (or Red, Green & Blue) and these two colour models work on separate ideas.
In short, CYMK works on 4 different colours, whereas RGB works on 3 colours. The reason for this is that your monitor has tiny dots of colour colour pixels, which are individually made up for 3 different coloured dots which are, you guessed it, red, green & blue.
CYMK on the other hand is based on the four different colours which are printed onto the paper which can make most colours easily (although not ALL colours are possible with CYMK that are possible with RGB).
The Additive Model
RGB is known as the additive model because it works on the assumption that you are you working from black to white – this is why when you have 0% red, 0% green & 0% blue you get black and on the flip-side 100% red, green & blue you get white.
This came out because the screens are naturally black and the pixels expel light in the relative wavelengths our eyes can see.
The Subtractive Model
CYMK on the other hand is known as the subtractive model because it works from white through to white (with 0% of all the four elements of CYMK being pure white). As you can see, this is completely opposite of RGB as you start to add colours, you get darker and darker colours, eventually leading to black.
This is due to the fact that printers work on white paper stock, and the inks are added to the surface which reflect different wave lengths based on which ink(s) are present and in what saturation.
So why don’t they look exactly same?
This is due to the fact that converting from one colour model to the other may not produce an exact match. In fact, this is partly because the RGB colour model can theoretically produce more hues and colours than the CYMK model, so producing an exact match is virtually impossible.
In fact, the RGB model can produce up to 16,777,216 different colour hues and shades (although distinguishing between two closely related shades with the naked eye is fairly hard if not impossible) whereas the CMYK model can only produce around 10,000,000 different shades.