if printers use CMYK and TVs use RGB why cant we use one ?

CMYK is inferior to RGB since that it can produce 60-70% of what RGB does.
Secondly , you get less per cartridge each year . and they are super expensive.
RGB : is the format used in TVs and LED.

the differnce http://www.google.co.uk/imgres?imgurl=http://www.mobiliodevelopment.com/wp-content/uploads/2012/04/RGB-compared-to-CMYK-colors.jpg&imgrefurl=http://www.mobiliodevelopment.com/rgb-cmyk-concepts-and-differences/&h=360&w=360&tbnid=pW6T8zGv5tvg6M:&zoom=1&docid=6-dfrvq2H_RQCM&ei=ngjzVPyUMcnzUICdgOgJ&tbm=isch&client=tablet-android-samsung&ved=0CEQQMygcMBw

I've got a question, why are you using a printer in 2015?

RGB is better. Websites and games use RGB, so it would be easier if everything was in RGB, however, if everything was RGB, when printing, black wouldn't look black.

CMYK only works for printed things, as red, blue and green are the primary LIGHTS, but cyan, magenta and yellow are the primary PIGMENTS (and black to save ink), so when you try to print in RGB you can't make some colours.

Therefore, things are good as they are now, RGB for lights and CYMK for prints.