According to Blizzard’s Mike Morhaime, the company simply “didn’t find the fun” when they performed a re-evaluation, citing that the game itself was way ambitious, although it did not “come together” as a cohesive work which one would want to play. Apart from that, there were some consternations among the developers that rolling out Titan would continue to shape themselves as an online RPG company, which is not the direction that Blizzard is heading, since they intend to roll out “great games every time,” and that is an objective that will require the switching of genres from time to time.
This is not the first time that Blizzard has stopped development on a new title because they figured out it will not really be able to work, never mind that the game is close to being completed, such as StarCraft Ghost and WarCraft Adventures. Perhaps this is the reason why the games that actually do make it to the market happen to be huge hits for years to come.