PO43- and NO3- are always bound to a cation, in this case potassium(K+). See Common Compounds of Nitrate NO3 -- EndMemo
. This is a list of common salts of nitrate, as you can see many are toxic to fish in the levels required to produce a rise in NO3- in the water. The same goes for phosphate.
K+ compounds(e.g. KNO3, KH2PO4, K2SO4) are the most common simply because they introduce potassium(a relatively safe cation) into the water that plants rapidly uptake. This likely leaves an excess of potassium, which is relatively non-toxic and removed with 50% weekly water changes(per the EI method).
Examining KNO3 closer for example we can see that K+ and NO3- are added in roughly a 2:3 ratio, i.e. in a 100gm sample of KNO3 there will be roughly 60gm of NO3- and 40gm of K+. Repeated for KH2PO4, we see an approx 3:7 ratio. And finally SO4 is closer to 1:1. Applying this to EI(dosing only KNO3 and KH2PO4) over a 1 week period 29ppm for nitrate, 8.7ppm for phosphate is accumulated. The K+ accumulated is 36ppm. This is assuming no plant uptake.
This level of potassium accumulation is somewhat negligible when we're targeting NO3 and PO4 levels which are much more sensitive and contribute to algae growth in the wrong levels. Its just not economical to dose them individually(as K, N, or P), considering potassium exists as a highly reactive metal, nitrogen as a gas and phosphorus which is never found as a free element, buying them in elemental forms would be a waste of money.
Hence why the popularity of KNO3, KH2PO4 and K2SO4, which are relatively cheap, easy to obtain and non-toxic compounds at the recommended dosage levels.