The Pokemon Company makes a set, and they make a bunch of copies. Well when a new set comes out they don't really make much of the one before, and the latest set you listed was "Vivid Voltage". Since then more sets came out (exactly 15). So the Pokemon Company doesn't really make to much of older sets, when a new set comes out they kind of over produce it, and then they don't care about it anymore.
You can still find the sets that you listed, but as you said, they are harder to find.
Hope I Helped!
(Also my source is experience cause I collect the cards)