When we work the psc, the stitch afterward is anchoring the ch3 down to form the picot.
If a psc is followed by a special stitch however, that special stitch prevents the proper anchoring of the picot st. (pic. 1)
Even after making a stitch following the special stitch, there is still too much of a gap to properly define the picot. (pic. 2)
The variation of the picot single crochet makes sure that the picot is anchored in place, so that a special stitch can be made without loosing the integrity of the picot. (pic. 3)