The problem is that lowering pH causes the soap to revert to the free fatty acid.

Na-CH2-(CH2)n-CH3 (Sodium soap)  +  C6H8O7 (citric acid)  —>  H-CH2-(CH2)n-CH3 (free fatty acid)  +   Na3-C6H5O7 (Sodium citrate)

As you know free fatty acids aren’t soluble in water, nor they clean as soaps do.
For lower pH levels, you’d need synthetic surfactants because they don’t decompose.
Sulfates, sulfonates, sarcosinates, isethionates cling to the Sodium atom “tighter” than soaps do. And in nonionic surfactants there’s no Sodium (or Potassium) atom to be lost.
Hope this helps.