Let me make one thing clear. I love Elizabeth Warren. I think she is one of the two best senators in Washington (with the other being Bernie Sanders of Vermont). She is doing a great job for the people, and someday might make a great candidate for president. But 2016 is not her time.
The thinking probably is that since Barack Obama defeated her in 2008, there is a lot of anti-Hillary sentiment in the Democratic Party -- and that those Obama supporters are now looking for a new champion to defeat Clinton. That is simply not true. While a small minority of Obama supporters (primarily from the party's left-wing) might like to see a more left-leaning candidate oppose Hillary, most do not. They were pro-Obama, not anti-Hillary. And they now believe that Hillary deserves her chance, because of the class way she handled defeat in 2008 and her excellent service as Secretary of State for President Obama.
The truth is that if Hillary Clinton wants the Democratic nomination in 2016, it is hers. Several polls have shown this, and now a new one verifies it again. It is the NBC News Poll done between November 7th and 10th of 1,003 nationwide adults, with a margin of error of 3.6 points (5.5 points among only Democrats and 5.8 points among only Republicans). It shows she has the clear support of at least 66% of all Democrats (and I suspect that would climb after she declares she is a candidate).
Elizabeth Warren is not a stupid person. She has to know she could not beat Hillary in 2016 -- and that her running would only siphon off a few voters and might hurt the party. For now, she is far more valuable in the Senate, where she serves as a watchdog on the excesses of Wall Street.
And even if he was to somehow convince enough Southern, Midwestern, and Western Republicans that he would be the best nominee, he probably could not beat Clinton in a national race. In fact, the area of the country he is the strongest in (the Northeast), is where he would do the worst against Hillary Clinton. He probably is the GOP's best hope, and would do better against Clinton than many others in making it a closer race -- but as the wise men say, close only counts in horseshoes and hand grenades. It won't get you into the White House.