Why it is difficult to synthesize peptide with FITC without the Ahx linker?

If FITC is directly modified at the n-terminal amino group, the Edman degradation reaction occurs in the cleavage process, as shown in the following figure. Edman degradation is the process of determining the sequence of amino acid residues from the N terminal free of a polypeptide chain. N-terminal amino acid residues were modified by phenyl isothiocyanate, and then the modified residues were cut from the polypeptide chain (the amino acids directly linked to PITC), which were identified by chromatography, and the remaining polypeptide chains (with one less residue) were recycled for the next degradation cycle.