Officially, it's cancelled - but creator Tim Kring is talking to NBC about making a TV movie or miniseries that would wrap up the plotlines.
Like most cancelled shows, the simple answer is low ratings - however, the TV writers' strike is also partially to blame. The strike affected the 2nd season of Heroes, meaning only 11 out of a planned 24 episodes were made. Any ratings momentum from the 1st season was lost due to the strike.
Yes, it ended abruptly without a last episode.
Yep. Ratings began plummeting around season 2 and never really rose to season 1 levels during season 3.
Yes. It was one of those that went from the top to below bottom in just a few weeks. I did not get to see many of them so I'm unsure what happen. Try www.tv.com
Heroes cancellation was announced around May 15th, and it was being considered whether there was going to be a movie or a mini series to wrap it up, but, although not officially, its been said that that possibility is no likely.
It was most likely canceled because of its decreasing ratings since the writers strike at the end of season 2, "less than 5 million viewers tuned in for February's Season 4 finale, down from about 17 million in Season 2".
For more info check my sources, hope it was of help ;).