"RCA clean" solution is a 5:1:1 mixture of DI water, ammonium hydroxide and hydrogen peroxide.
Why the ready 5:1:1 solution is not stored for long term, but instead has to be prepared each time immediately prior to usage?
Hydrogen peroxide is not a very stable substance. Over time, and however you store it or however carefully you prepare it, it will decay. So solutions of a given concentration need to be prepared fresh when you want to use them. You could try to store them but you won't know whether the concentration is still suitable for whatever you are going to do with it at some later time. And, the decay may well generate gas and other side-products of oxidation what may interfere with the use and may be unsafe.
Hence you would make the mixture fresh every time when you want to use it.