After searching for a short moment:
I have understood that there were mainly:
- terrestrial TV channels: KBS, SBS and MBC
- cable TV channels (paid ones): tvN, JTBC
This article in late August 2019 (https://en.yna.co.kr/view/AEN20190826005200315) mentions the reasons of their struggling and financial loss:
1) falling viewership ratings in weekdays dramas for terrestrial channels (from 2-digit to 1-digit) and for news programs:
"About a decade later, however, the broadcasters, which have been struggling with falling viewership ratings, decided to remove 16-episode series from their prime nighttime slot on Mondays and Tuesdays as part of their cost-saving efforts. Weekday dramas will be broadcast only on Wednesdays and Thursdays from now on.
The move came after a series of failures of big-budget weekday dramas in the first half.
The article does a comparison with weekdays audience (before/now and networks):
-Before: 40% audience easily for TV networks
-Now: < 10 %
-News sector: 18% before, 11.5 % now for KBS (same trend for SBS and MBC)
2) Competition and cable channels with 2-digit viewership ratings:
“Separate data released from Nielson Korea, the combined viewership ratings of three major terrestrial broadcasters (KBS, SBS and MBC) reached 62.23 percent in 2000 but nearly halved to 33.4 percent in 2018.”
“Over the 18-year period, the corresponding figures of paid TV channels, including tvN, soared to 17.68 percent from 2.03 percent.”
Ex: “Guardian: The Lonely and Great God” by tvN and the black comedy “Sky Castle” by JTBC overwhelmed terrestrial channels’ contemporary programs with ratings of 20 percent or higher.
1. and 2.
Why does this competition gain more viewership? The plot.
Dramas on terrestrial TV channels have failed to measure up to viewers’ expectations due to their corny story lines and formulaic plots, while cable channels have picked up experimental and flashy shows and created solid fan bases.
3. Change of habits (Millenials don’t watch content on TV channels + revenue/cost from an advertisement on TV + viewership ratings on TV and how to record viewership rating on the internet, the medium of communication we mostly use?):
Experts, however, said a decrease in terrestrial channels’ grip has been long anticipated amid a change in the media environment as a growing number of people rely more on new kinds of platforms like YouTube, Netflix and social media rather than turning on their television sets.
Back in 2011 (8 years ago) and 2015, it was the same matter and other reasons were:
4. Cost compared to revenue: the plot again
“the problem is that drama production costs just keep rising, and advertising for low-rated shows isn’t bringing in enough revenue. It probably doesn’t come as a surprise that the highest cost goes to the lowest-rated shows: prime time weeknight miniseries (16 to 24 episodes), which are still the marquee programs for the Big 3. Yet the reality is that long-run weekend or daily dramas are far more cost-efficient and have bigger viewership to boot.”
The author’s comment for removing Mon-Tues drama slot on SBS:
“On one hand, it’s too bad that after having a year of extra choices, we’ll be losing one slot. On the other, sure makes no sense continuing something when it’s a losing proposition.”
I think they cut down on TV production from Mon-Tues nights because it was where a big part of the financial loss was coming from with not enough TV viewers and TV revenue. It was the root that lead to this decision.