Part of the reason for the consensus is that Palin, unlike everyone else who delayed entry (Bachmann, Jon Huntsman, Rick Perry, potentially Chris Christie), Palin had no reason to delay. If she were going to run, there would be no reason for her to wait until everyone else had entered the race. In fact, she had very good reason to get in on time: to keep Bachmann out. If Palin had intended to run, her delays not only frittered away her time, they invited Bachmann into the race; Bachmann would almost certainly split her constituency.
There was also some "crying wolf": Palin's bus tour during the summer, her appearance at a pro-Palin movie premiering in Iowa, etc. The media covered Palin, and she didn't make any movements toward running. At last, they tired of being manipulated. Unlike polls earlier this year, current polls rarely include Palin.
It's fair to conclude that Palin decided earlier this year that she would not run for president. There were early signs, as she let hints slip in bits and pieces in various interviews. Palin recognized that general election polls and Republican primary polls showed she was too polarizing. She was very unlikely to win the nomination, and almost certain to lose the general election. Since she could not win, there was little point in running. There was plenty of downside: She would lose the opportunity to make money, and her reputation would be tarnished.
But, once again, political commentators are suggesting Palin might indeed jump into the race. Some have said that her schedule in September looks like a candidate's. Others believe she will announce her candidacy on September 3rd. What could explain this?
Partly these rumors can be chalked up to wishful thinking. The media finds it easy to cover Palin, and if she actually did run for the presidency, it would sell a lot of newspapers. Liberals are keen to see Palin run because they enjoy attacking Palin, they like that Palin's image casts the Republican Party in a negative light, and because they would like to see Barack Obama beat her in a general election. Some Republicans would also like to see her run. Palin fans, of course, want her to run. But supporters of Perry and Mitt Romney also want to see Palin run, because it would split Bachmann's vote and ensure Bachmann could not win Iowa.
Still others have floated Palin's name because they want Chris Christie to run, and it's easier to write about that when they can disguise their intention by talking about how "other candidates" might run (e.g. Paul Ryan).
Ultimately, however, the rumors about Palin persist because she still could run, if she wanted to do it. Unlike Mike Huckabee or Mitch Daniels, she has never made a declaration that she will not run. And although Palin did earlier decide not to run, she could change her mind. She may have seen Bachmann doing well in the (Iowa) polls and stealing her position as the "Alpha Female" of the right wing. Palin decided not to run because she believed she couldn't win. She also thinks Bachmann can't win. And if Bachmann can do well in Iowa polls and win the Iowa straw poll, perhaps Palin would be motivated to enter the race, purely out of spite. Time will tell.