There happens to be a candidate who relies on small donations - if he can do it, why can't she?
In all fairness, not everyone can raise that kind of money via small donations. Presidential campaigns are super expensive and at this point, it's not even a given that Sanders can raise the necessary capital to run a strong, competitive campaign that also doesn't neglect downballot races
(I would personally never vote for someone who didn't assist in other races).
It takes a certain kind of person to have such an appeal that they can raise such money, and just because someone doesn't have that appeal doesn't make them a bad candidate for president. They could end up being a great president but still lack the appeal that they would have needed to run a campaign on small donor support.