Why was Terriers canceled after its first season?

IMDB ratings aren't a good indication for a show's chances to stay on the air. Not only are they unreliable due to the inherent selection bias (the people who vote are usually people who care enough about the show to go look for it and vote), but even if it was a good, relatively objective measure of quality, it wouldn't have much to do with cancellation.

Networks make money by selling advertisements during shows. Advertisers pay for shows that bring a lot of eyeballs to watch their ads. Thus, networks keep shows that have high ratings. Quality might be linked to ratings, but it's certainly not any sort of indication.

According to Wikipedia, quoting the site "TV By The Numbers", viewership for the show started at 1.41 million viewers, or 1.1% of the viewership - not particularly high ratings to begin with, and even fewer, 0.5%, among the 18-49 year-old demographic, considered a lucrative one (as in, they buy things that are advertised).

By the season finale, that number had dropped to 0.5% overall, and 0.3% in the target demographic.

Apparently the managers at FX decided that the show wasn't going anywhere, despite critical approval (and high IMDB ratings), and that the money it cost to produce wasn't justifying the returns on investment from advertising, and it was cancelled.