Sanders is establishing himself in the early primary states. The rest of the country doesn't know him very well yet. If he wins the nomination, given what he's done so far financially online, I think he'll raise plenty of money for the party. In the interim, that can't be anywhere near his top priority.

As well, there are a variety of reasons affecting what has happened with this so far:
1. Sanders doesn't do many fundraisers or ticketed events like Hillary that tend to attract folks who can afford to give to the DNC.
2. Hillary's donors include the Wallstreeters and wealthy folks who like to buy politicians (including Hillary) who have excess money to give to the DNC. Sanders donors typically send him $30 and can';t afford to give to the DNC
3. As the DNC is essentially an extension of the Clinton campaign and has treated Sanders unfairly, Sanders supporters giving to the DNC would arguably be investing in an entity that is clearly against Bernie - so they'd be effectively or symbolically investing money that went against their own candidate at this point in time and therefore, investing against themselves. The Sanders campaign owes the DNC absolutely no favors. Their bias for Clinton that goes against their bylaws is a national disgrace.
4. Hillary has a super PAC that can help raise for the DNC while Sanders doesn't.
5. Sanders has little support from the established Democrats who help bring in DNC money because it's in their mutual interest (for themselves and their party) to do so.
So let's not be naive about why the Sanders campaign is where it is on DNC donations.