Because based on atomic structure, in hydrogen chloride chlorine has more
tendency to gain electron of hydrogen and produce H+ and if hydrogens in phosphoric acid give their electrons to oxygens, there will be repulsion among non-joint electrons pair. Also that is why H3PO4 has acid ionization constant \(K_{a} = 7.25*10^{-3}\) and HCl \(K_{a}=10^{3}\).
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