The number one reason is explained in "News Is What's Now".  The story must be timely.  Sometimes you will get lucky on a slow news day when the newsroom does not have much to choose from. 

There are 3 key ingredients: 

1. First and foremost, a story needs to be easily told through video and sound, otherwise it will be considered a better story for the local newspaper.  Think of a 2 x 4 flying through a window at 40 miles per hour.  Great video and sound right?  A window company once baited my television station several years in a row with a similar stunt.  The company would fire the piece of wood through a standard window, proving that it could not withstand the force.  Next, the company would repeat the stunt, but replace the standard window with one that was considered "hurricane resistent".  In the end, we showed video that was fascinating to the viewer, while heavily promoting a company's product without charging them a dime.     

2. A news story must also have what is commonly referred to as "WIIFM".  What's In It For Me.  Think of "Me" as the viewer who will sit and watch the news story.  Are you going to make them faster, stronger, smarter, healthier, wealthier, skinnier or more beautiful?  By educating the viewer, are you going to protect them from danger?  Much of the decision-making in newsrooms is centered around this type of thinking. 

3. Make sure you have someone in mind that can serve as a "CCC" or a Central Compelling Character.  This might be an average consumer who praises your product or service, or has benefitted in some way.  Finally, provide as much information that will explain your product or service and its daily impact on the community.  News reporters like numbers, as it pertains to money, statistics and comparisons.  Be careful, though.  Making it easy for a reporter to tell the story is inviting, but remember to allow the journalist to have editorial control.  Journalists like seeking truth, and prefer to tell a story the way they interpret it.  

The more that your news pitch resembles a commercial, or appears to be the result of your efforts to push an agenda, the less likely it is to hit the air.