Endorsements usually do not mean much, and have multiple, and sometimes unclear, motivations. Sometimes just old history with the person trumps other considerations such as nation and party.
Those hiring people for jobs do reference checks with former co-workers and former employers. Why? They have the best information about how the potential employee performs at their jobs, and provides the best prediction of how they will perform in their new job.
United States Senators are the only ones who have worked with both Obama and Hillary in the same capacity and in the same setting. One might have expected the Clintons' long history that antedated Hillary's Senate career with many of these members to have won their support. Moreover, they have worked with her longer in the Senate because her tenure is longer, as the Clintons continually point out. They know her better.
These are the people who can best judge the claims to judgment, competence experience, likely success and unifying capacity. Making any choice in this situation is difficult because all know that they are going to have to work together in the future with one of them as a colleague and the other as president.
By 18-13 United States Senators have endorsed Barack Obama.