Choosing mud to sling

Why does Senator Obama highlight aspects of Hillary Clinton's history like service on Walmart's board (is that a crime?) when there is so much else to choose from. 

In 1992 Ms. Clinton insulted many American women who choose to stay home and raise their children (remember the "bake cookies" comment?).  Then we learned about her involvement in FileGate and TravelGate. Later Ms. Clinton made preposterous statements to the press about a "vast right wing conspiracy" when asked about her husband's peccadilloes which were already chronicled.  One could go on and on.

Now, she criticizes Senator Obama for the quality of clients he represented while he was in private practice.  Ms. Clinton would likely prefer to discuss her board memberships, as opposed to other issues that stained her dossier.