What are shampoo designed to do anyway?
Shampoos are designed to clean your hair and scalp. Dirt that could be sitting in your hair includes pollution and particles from the environment, old/rancid sebum, sweat, smoke from restaurants and clubs, flakes of old/dry skin (eww!), greases/oils from products and dirt from the things your head touches.
Why can't a conditioner do the same job as a shampoo?
The surfactants (cleaning agents) that can be included in a conditioner without destabilising it are not capable of clearing all this dirt out.
Why are clean hair and a clean scalp necessary?
Clean hair is necessary because it’s more aesthetically pleasing and is easier to style. A clean, pliable scalp is necessary for healthy hair growth.
How do ingredients in shampoos work differently to conditioners?
Shampoo works by lifting dirt off your hair and scalp long enough for it to be rinsed out by water. On the other hand a conditioner contains lots of "cationic" ingredients - these are ingredients that are strongly attracted to hair and stick to it. Ingredients in a conditioner are designed to stick to hair even after it has been rinsed so that they continue to condition the hair long after.
You simply cannot wash dirt out using a product that is designed to stick to it.
I always compare washing your hair with conditioner to having a bath with lotion - you could do it, yes but a fair amount of dirt and oils would remain on your skin. The stickiest ingredient on my body tends to be my roll-on; it is sometimes so stuck on that I have to rinse my armpits twice with body wash to get it off fully - trust me, washing my pits with lotion would not get the job done! (Okay, I'm sharing too much info already, let's move on…)
So, what's the bottom line on the whole no-poo thing?
Overall, if you go “no-poo” you need to be clear that some of the ingredients in both rinse-off and leave-in conditioners, even the “natural” ones, can only be washed off with the surfactants in shampoo so they will build up on hair. This will lead to all the problems associated with build-up like breakage, increased dryness and lack of moisture, difficulty in product take-up and so on.
If you feel that commercial shampoos simply aren't working for you then perhaps you should work on learning how to make your own shampoos using one of my DIY Shampoo Recipes so that you have the perfect shampoo for you.