Sanders's campaign started out with basically him and a small group of operatives (likely in the single digits) who hadn't been hired by anyone else. Compare that with the Clinton campaign, which had the institutional advantage of both Clinton and Obama's political networks to start, and was about as large of medium sized company by the beginning of this year. Is it any wonder that the Clinton campaign has been better connected and better organized?
Sanders campaign relies mostly on amateurs (a lot of former Occupy people, actually) along with paid staffers who the campaign only was able to start hiring this past fall once the online fundraising operation got going. Because the entire Democratic party and most unions basically decided to support Clinton before the nomination began, the Sanders campaign needed to build a campaign infrastructure from scratch. That's something even the Obama campaign didn't need to do because Obama always had at least some institutional backing - he'd given a major speech at the 2004 Democratic convention for goodness sake - even at the beginning.
Despite those disadvantages, the Sanders campaign has managed to gain the support of at least 40 percent of Democrats if you go by national polling.
The Real Clear Politics poll aggregator has him with an average of 42.5 percent support (Clinton +9)
http://www.realclearpolitics.com/epolls/2016/president/us/2016_democratic_presidential_nomination-3824.html#polls
and the pollster.com aggregator, which incorporates more polls so is better, also has with with the same level of support and down by a slightly smaller margin.
http://elections.huffingtonpost.com/pollster/2016-national-democratic-primary
Looking at the trend-lines on both aggregators, Sanders still appears to be gaining ground on Clinton as well.
In addition to that, Sanders does better in every poll against every Republican candidate than Clinton. Many people like to dismiss this point by saying that general election polls at this point aren't very predictive (true-although it gets harder to make this point once you get into April), that Sanders hasn't faced Republican attacks that will turn people against him (somewhat true, but its highly debatable how much this will matter), and that Clinton has faced every attack the Republicans have thrown at her (untrue). Nevertheless, its hard to ignore this pattern because it occurs across all polls, lines up with other measures (favor-ability ratings), and seems to reflect some of the crossover support Sanders got in places like Michigan.
Most importantly, Sanders is getting the support of the vast majority of millennials who are voting. Considering millennials are the largest age demographic and will play an increasing roll in politics going forward, the Democrats can't afford to ignore that. If this primary were held 4 years from now, Sanders would have won Iowa, Massachusetts, Illinois, Missouri, and maybe Nevada. In addition, the he would have won the states he did win by wider margins and lost the states he lost by smaller margins. If this primary were held 8 years from now, Sanders would have won handily. That is how generational changes in an electorate work. Not only does the candidate winning the larger share of younger voters gain as those voters become a bigger portion of the electorate, the candidate gaining most of his or her support from older generations loses that support as older cohorts age out of the electorate and stops voting.
Sanders won't win the primary, but he still has every reason to stay in the race to demonstrate that support. Sometimes, candidates run in elections with goals other than winning, especially primary elections. If Sanders were to drop out now, many would do what they are already doing: try to write off the platform he is running on as a flash in the pan that won't amount to anything. To prevent that, I hope Sanders does what he appears to be doing: consolidating his support in the hopes of fighting for the Democratic Party to represent at least some of what he wants at the convention. Considering Democrats will need Sanders supporters to vote for them during the 2016-2024 period, that is only fair. That is how primaries and political conventions work.