There are, presumably, a finite number of people who are willing to give $2300 or $4600 (maximum allowed) to presidential candidates. I get the sense that the candidates are spending their time courting the big donor fundraising networks, which makes sense I suppose, but given that they're sucking up all that money so early, at some point I imagine that well will run dry. Maybe I'm wrong and there are more $2300 checkwriters than I'm aware of.
...to clarify, there's more money out there than big donor money. Us little people can be inspired to give. What I mean is that if candidates suck up all the big donor money early, then six months from now there will be no more big donor money left to receive.