In an interview with student journalists, says:

“I’ve just been involved in politics for a very long time and I just don’t think that would be the ticket…The only time I saw something like this come together was when I was your age, when I was in college, and John F. Kennedy chose Lyndon Johnson.”

WashPost suggests that comparing the two to Kennedy and Johnson indicates Pelosi believes their personalities/approaches are too different to form a ticket.