Moses Herzog is a man obsessed with the past. He is unable to conquer this obsession and, most likely, he is unwilling.
Ramona offers Herzog a new and fresh future, even though she might also offer some complications having to do with her own past. Nonetheless, Ramona can be seen as representing a break from the past and this is a break Herzog cannot accept.
Examples his obession are many. Herzog's letters, his fixation on Madeleine, and his impulses to visit his family are all evidence of this need to maintain the past and even bring it back.
For Moses Herzog, a disastrous confrontation with the past is preferable to a break with that past. This may be true for a number of reasons, one of which is Bellow's interest in the (potential) emptiness of modern life.