There are several reasons for the show going under...
1) The defenders was on at the same time, while terriers started a week earlier a lot was unknown due to the poor marketing campaign... The reason why this is important is simple... Terriers is very linear, if you missed an episode you were out of luck... A lot of shows do bad with linear story lines as if you dont keep up someone just getting into the game at episode 5 is out of luck. Defenders however had no such issue.
2) The commercials were as noted shitty... Along with the name, neither tell you much about the show, or what was going on. While not every title is obviously going to be a winner, vs say the defenders... Thats what they were Defenders, so it made the demographic a lot easier to note, it also made the central story easier to advertise. That was a big issue too about terriers... They were PI's who got 1 SUPER job and 13 epsiodes about basically figuring it out. This is very hard to advertise (if you watch the ad's that was the point of them, but this is only apparent if you watch the show)
3) IT WAS NOT ON ANY PREMIER LIST I SAW. My girlfriend had to do a paper on a show during the fall season. I told her to do this show as it looks interesting and I told her sure to be canceled. When she linked me to the page she was using for the paper, Terriers was not listed. This was a common problem, my mom had no clue about it. Co workers knew nothing, friends, etc. This was like say better off ted if no one is talking about it, obviously its not really out there.
4) The end of the season kinda marked the end for an easy story... It felt like they knew it was over from the start and went the way of happy town. While the story keeps going you find out who the magic man was (or in this case behind it all), and the rest is just "sorry network sucks"