josef
Tochachah is i believe a D’Oraisah.
the Halochos of giving Tochachah are D’Rabbonim
i would SPECULATE that at the time the gzairos were promulgated, the general populace was only familiar with the Torah b’ Csav, there were many periods in our history that this was the case.
Therefore it would make sense to institute that for a meforishah mitzvah, Tochachah should be given regardless of whether or not one would listen. Those Mitzvahs that were derived by Chazal, however, would be less known and therefore Tochachah should only be given if there was a reasonable chance they would believe and listen to you.