No surprise here. Edwards has had one thing on his mind since his exit from the primary, and at a time the Obama/HRC battle had barely begun: a second shot at being VP. As HRC has been pushed closer to the center, it has become plainly obvious that Obama's brand of liberalism fell directly in line with Edwards' loony, patronizing populism. So at first blush, although it seems surprising that Edwards sat out this long, he clearly wished to avoid endorsing the wrong candidate.
Then again, he's promised to dedicate the rest of his life eliminating poverty, one $400 haircut at a time. So perhaps he's been busy.