There is a reason the Planned Parenthood video got zero coverage initially, that's because the whole video was B.S. and even with the heavy editing nothing that was talked about was actually illegal. Everything that happened afterwards like the potential shutdown of the Government over the video and a Presidential candidate out right lying about what she saw in the video is news.
The Clinton email story got coverage from the start, do you want ABC, CBS and NBC to preempt the rest of their schedule and bleed into their affiliates programming time to cover a story that is still unfolding? On a daily basis there isn't that much to cover so it's going to be a quick mention on the nightly news, the cable news channels are the places to go for more coverage(mainly people arguing over it).
Also who's watching the network nightly news anymore? People waiting for the lottery numbers and senior citizens who keep their tv tuned to channel 2 waiting for Bill Burns to come on.