They lost 2.1 million viewers between the first week and the second week, another million viewers in the two weeks after that, and then returning for season 2, they lost another million and it just kept going downhill (stats are here
https://en.wikipedia.org/wiki/List_of_Dallas_(2012_TV_series)_episodes sources are in the references below).
I'm no expert, but I have a feeling the new show alienated a bunch of original, but not overly dedicated fans (e.g. people who watched it and enjoyed it in the 80s and were interested in seeing what the new show was about, but they were never ones to revisit the original on DVD, TV reruns or come to forums like this) when it became obvious even in the first episode that it was related, but not really modelled on the original show. For example, JR wasn't doing the same dastardly deeds he was known for (well, he was, but not quite in the same way); Bobby wasn't doing anything of any interest and his wife was a newbie without any real spark; Sue Ellen was barely around; and the old, familiar, thriving Ewing Oil was no more.
Additionally, if you did come into the show expecting something similar to the original method of storytelling, the newer, fast-paced storytelling may have been unappealing. For many too, crime dramas aren't really a genre they watch; I know personally for me, if the show hadn't been
Dallas and featured some originals I wanted to see, I wouldn't have continued to watch; many of the storylines (not just the drug cartels) were just not of any real interest to me, so for less dedicated fans (as explained above), once the initial curiosity of seeing what became of some of their favourite characters of the 80s was satisfied, there was little else there to hold their interest.
In regard to the content and characters, the retconning was too much for many of us to swallow with ease; the way a number of characters were written wasn't love to hate, it was just general dislike, without real feeling; the show was supposed to be about oil (or at least alternative energy) but very rarely was oil or even business a topic, drug cartels are a whole different genre; the family drama was focused on the wrong families for people to care (it's ok to develop other families, they did it on the original with Sue Ellen's mother and sister, Pam's mother, brother and sister, Clayton's family etc but character development of those people didn't come at the expense of the Ewings). Just a few examples.
As well as that, although not a lot of the general TV watching population also participate in forums, go to Q&A sessions or interact with the cast on social media, I do believe the way some of the external promotion the cast did was a disservice to the show. Cynthia Cidre admitting she and the team of writers had "collectively", but not individually seen the entire show was just horrible to hear; the use of Linda Gray to promote the show prior to airing was a huge letdown when it came to watching looking for Sue Ellen's storylines; the OTT teasing that Pam could return, only to have Victoria Principle squash the idea entirely (not her fault, I don't think she was wrong in clarifying that she wasn't returning). Again, just a few examples that spring to my mind.
I do think they did some good things, it was nice to see the cast back; JR's funeral was well written; Josh and Linda played well off each other; it was nice to see the Southfork exterior shot in HD etc. For casual viewers though, there was very little that was great and when a show is competing with so many other shows on other networks, that's not a good thing.
That's my two cents.
