There are plenty of reasons why. First off, I don't buy it. I don't buy her. I think she's a political opportunist who is for whatever will get her the most support. She didn't waver once in voting for the invasion of Iraq, and didn't say a word until it was politically convenient to start criticizing the President. Her tears may well have been genuine, but it's hard to take it seriously when it was so well timed. Her campaign's antics (such as the staffer who brought up the attacks the Republicans would use against Barack - basically attacking him by proxy) are not what I want from a presidential candidate.
Attacking Barack Obama's campaign for keeping her comments regarding the Civil Rights Act (she first said that MLK worked for it, but it took a president to make it happen, then rephrased after she realized how it could be interpreted) are apparently better attributed to the Press (according to this Moderate Voice Article). President Clinton's attacking the credibility of Barack Obama's opposition to the war, particularly in light of Senator Clinton's clear vote for the invasion just throws gasoline on the situation (also read on The Moderate Voice here).
Perhaps all this would be acceptable, if it weren't for the fact that Hillary is no less polarizing than the current Executive, if not more so. What this country needs right now, more than anything else, is a President who knows how to listen, how to work with BOTH political parties, and how to get America to get along again, with ourselves and with other nations. Ron Paul might be able to do that; Barack Obama, perhaps, but I'm pretty certain Hillary cannot.