Probably because it's a great money spinner for HP..:-)
My Canon uses four cartridges: black, cyan, magenta and yellow.
They cost about £2 each from CPC in Preston with the double capacity black version about 50p more.
These are by Jettech but, until recently, CPC sold the JR equivalents at the same price; only difference was that there were TWO cartridges in each pack.
Even more fascinating was that JR's own website listed the cartridges at more than £6 on its own website and that was for one, not two, cartridges.