It's not that. Sanders has a reputation for refusing such events. And that's why he has one of the lowest net worths in Congress. In his own way, the old fellow has a sense of honor, and that's what makes him so attractive to the Occupy Wall Street types.
Sanders would make a good trustee for a bank, credit union, or something like that. But if he were president of the bank, his unrealistic view of the world would run the bank into the ground in about a week.
A week? That long?