Hi,
Because every other possible fert is nowhere near as important as NPK. There are lots of other elements that are completely ignored by EI dosing schema. We do not address Sulfur, for example, or Cloride, or Nickel. At the end of the day the dosing scheme was developed in order to be easy and accessible, and to cover the most important nutrients that plants require for optimal health. Calcium and Magnesium were considered adjuncts and are normally available in sufficient quantities just as are S, Cl-, Ni and other, more obscure micronutrients. Deficiencies in these are never as problematic as are deficiencies in NPK and Fe.
You need to step back and think about the context under which Barr developed this dosing scheme. At the time, NO3/PO4 were boogiemen, and people continued to suffer great difficulties with plant health because of their refusal to recognize or acknowledge plants' dire need of these essential components. EI dosing addresses these basic needs and resolves the issues associated with poor performance. Only very rarely do we ever see a Calcium or Sulfur deficiency, for example, so really, , this was not really a focus of the dosing scheme, although, yes, this can possibly occur if using RO water without remineralizing it, which is not a wise thing to do anyway.
Therefore, if you are using RO water it should definitely be remineralized, either with tap, with GH Booster, or with one of the (more expensive) commercial remineralizers. EI does not really want to be responsible for fixing this basic flaw in plant and animal husbandry.
Assuming you have followed the remineralizing procedure, you can now follow the basic EI dosing scheme. Adding more of everything can certainly boost performance, can enhance colors, boost growth rates and can even stimulate flowering in some species, but these special enhancements are not the objective of EI. The objective is to fix the most important and basic needs of plants. This doesn't mean that we avoid suggesting to add these adjuncts, quite the contrary, but these are less important than getting people to realize that they should, first and foremost, always address Nitrogen Phosphorous and Potassium in their dosing program. NPK are 1000X more vital than Calcium.or Magnesium or any other micronutrient we can think of.
Cheers,