Here's why:

* Ionic compounds: Iron(III) phosphate is an ionic compound, meaning it's formed by the electrostatic attraction between positively charged iron ions (Fe³⁺) and negatively charged phosphate ions (PO₄³⁻).

* Solubility rules: There are general solubility rules for ionic compounds. One rule states that phosphates (PO₄³⁻) are generally insoluble, except for those of the alkali metals (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and ammonium (NH₄⁺). Since iron is not an alkali metal, iron(III) phosphate is insoluble.

Note: While it's insoluble in water, it might dissolve slightly in acidic solutions. This is because the phosphate ions can react with the acid to form soluble species.