Here's why cult vampire series Moonlight only lasted for one season. While the late 2000s occasionally featured more vicious takes on vampires like 30 Days Of Night, this era put more of a focus on romanticizing the creatures. This is obviously related to the massive success of both the Twilight books - where a teen girl falls for a vampire at school - and the subsequent movie franchise. There was also The Vampire Diaries, which ran for eight seasons on The CW.
Entering somewhat into that fray was the 2007 TV series Moonlight. This series cast Alex O'Loughlin as Mick St. John, a private eye who also happens to be a vampire. The show started life as a movie concept that briefly had Bruce Willis attached to the main role, before morphing into a series that was originally called Twilight - which later changed for obvious reasons. CBS completely re-shot the original pilot after a bad response, replacing everyone bar Alex O'Loughlin to give it more of a youthful feel.
Moonlight cast Sophia Myles as reporter Beth and Shannyn Sossamon as Mick's former wife Coraline, who turned him into a vampire in the first place. Since CBS are mostly known for straightforward procedurals its was a fun novelty for them to have a vampire TV series, and while Moonlight didn't receive glowing reviews, it soon gained a cult following. Despite its early promise, it was confirmed in early 2008 the show had been canceled.
The run of the show itself was somewhat tangled, as Moonlight was originally picked up for thirteen episodes. By late 2007 the odds of a second season were looking good, though production was shut down because of the Writers Guild of America strike that lasted from November 2007 to February 2008. This meant only twelve episodes of the original order were filmed, and when the strike was finished, CBS decided to add four more episodes to the first season's run. It appears the network cooled on Moonlight after these episodes aired as their ratings were somewhat lukewarm, and it was canned shortly before the finale "Sonata" aired, which at least ended on a promising note of romance between Mick and Beth.
While efforts were made to ship Moonlight to other networks, with SYFY strongly considering the project, it was ultimately canned. In 2008 then CBS head Nina Tassler (via IGN) somewhat lamented the show's early death but stated the network would love to work with Alex O'Loughlin again, which soon came to pass when the actor accepted the lead in the 2010 reboot of Hawaii Five-O, which recently ended after ten seasons.