because pritzus is one of, if not the, biggest pitfalls of our generation. it NEEDS constant rehashing. and obviously the gedolim recognize this and rightfully stress this important topic.
talking about tznius is never too much, just like talking about loshon hora is never too much. if there was a big problem with many people being mchalel shabbos (like there is with people being mchalel tznius) I am certain our gedolim would endlessly be talking about the importance of shmiras shabbos.
btw loshon hora also gets many shiurim.