Possibly because the POH for the aircraft stated no slips with flaps deployed. Many do.
Probably though, he was just not a very good CFI. I know many sailing instructors who were really not very good sailors. I assume the same is true of CFIs.
i think slips on cessnas are prohibited with more than 20 degrees of flaps.....i don't have the poh handy, and can't recall off the top of my head.......
and, yes, you are right about some cfi's being good, and others not. my primary cfi was a great guy. i liked him a lot. the problem was he only taught what was necessary to get me passed my checkride. nothing more, nothing less.
the guy i fly with now, teaches as if he was teaching his own son. i like that teaching style better. even when i've flown with him not as a student, it ends up being a lesson anyway. great guy, and at this point, i almost don't want to fly with anyone else.