יהיו לרצון אמרי פי
The Rema (O.Ch. 123:6) writes,
The Chazan at the end of חזרת הש"ץ
does not say .
The Mishna V'rurah says the reason
is because he relies on the תתקבל
of the Kaddish, he will be saying
after ובא לציון.
(not to be repetitive)
The M.V. also quotes the של"ה and
the גאון who argue and hold he should
The Minhag is to say it quietly, not to
offend the Psak of the Rema.
*Possibly the same reasoning would
apply during Selichos at Shma Koleinu.
Since the Chazzan ends the Selichos
with the Kaddish w תתקבל.