DC's lack of representation in Congress is concerning for four reasons: 1) It denies Americans in DC their human right to be represented in governmental decisions that affect them. 2) DC has high populations of blacks and Hispanics, groups that have been targets of racial oppression in the United States. 3) DC is one of the nation's principal cities. 4) DC voters are overwhelmingly Democrats, indicating that the attempt to continue to deny them their rights is politically motivated.
Political Representation is a Human Right
Without getting into a book-length treatise on political philosophy, Americans widely recognize that political representation is a human right. It's part of the Universal Declaration of Human Rights. Countries like China and Saudi Arabia, who deny their citizens their human right to vote are properly criticized as dictatorial regimes that hold power by force, but lack democratic legitimacy. The US has launched wars on the pretext of restoring democracy (e.g. Iraq), and yet fails to ensure that the human right to vote is protected for all Americans in our own country in the seat of our federal government. It's the height of hypocrisy.
DC Has a Large Non-white Population
DC's population is about 47% black non-Hispanic and about 11% Hispanic . Anybody with a cursory knowledge of American history knows that blacks have faced centuries of oppression in the United States, from their complete dehumanization and transformation into private property (slavery), to the Jim Crow era with its lynchings, segregation and voter suppression, to the modern era in which social disparities are stubbornly persistent, and racism just won't die. Even if DC were 100% white, its lack of representation would be an injustice, but the injustice is particularly troubling given its strong connection to race and the oppression of racial minorities in our society.
DC is a Major American City
From an urban planning perspective, it often seems like cities don't get a lot of respect from the federal government. Funding for public transportation is usually hard to come by, with federal investment being much more heavily tilted towards roads. Roads are the bread and butter of transportation infrastructure for rural and suburban areas, but large dense cities like DC and many others need robust mass transit to function well. Dense cities with robust transit systems are great for the environment, leading to a lower national carbon footprint, to cite just one of many benefits of urbanism. DC residents pay their taxes like everyone else, but when it comes to getting something back from the feds, not so much, as the years of deferred maintenance on its Metro system attests.
DC Voters Overwhelmingly Favor Democrats
This is not a trivial point because it speaks to the reason why DC has had such a hard time securing the human rights of its people. For example, in the 2016 presidential election a jaw-dropping 92.8% of DC voters voted for Hillary Clinton. If DC were a state, it would have two Senators, who would probably end up being Democrats and a real voting representative in the House. If DC were reincorporated into the State of Maryland, Maryland might have more representation in the House and DC voters would have influence over the choice of that state's Senators and Representatives. Currently 9 out of 10 of Maryland's congresspersons are Democrats The political calculations are obvious. Republicans stand to lose power if DC gains statehood or to a lesser extent if it reincorporates into Maryland, so why would they support it? Yeah, why would a person support people, particularly people of color, having the right to vote? I mean, that would imply that we respected human rights or had some core principles as Americans that were more important than seeing our political team win.
Don't you feel great now? And don't even get me started on Puerto Rico and other U.S. territories which are similarly denied a meaningful voice in the federal government. The good news is you can do something. Contact your Representatives and (barf) President Trump to tell them the people of DC need their human right to vote respected (it may be futile, but at least you'll annoy someone on his staff).
 2015 American Community Survey One-year Estimates, Table B01003, via AmericanFactFinder.
 2015 American Community Survey One-year Estimates, Table B03002, via AmericanFactFinder.