There are a number of reasons. There are hundreds of thousands of claims. Since 2010 Congress has continued to defend the Social Security Administration's (SSA) local offices, and this results in claims processing delays. Sequestration has exacerbated this trend by cutting funds for local office funding more. Lastly, the government shutdown in 2013 has caused more delays.
Congress has systematically cut funding for local offices since about 2010. This has increased the processing time for an average claim that goes to hearing from 18 months to more than 24 months. This is a concerted effort by conservatives in Congress to discourage disability filings and to "wait out" claimants so that they give up or die.
Sequestration further delayed cases by cutting local offices budgets further. There is hostility to disability cases in Washington in spite of the cases being a small portion of government programs. Likewise, many of these claims are returning money people already paid into the system. Contrary to the mindless rhetoric of intellectually bankrupt Congress people, disability is not a handout, yet tax breaks to billionaires are. Logical consistency is not important to the illogical.
The government shutdown in 2013 delayed cases already waiting on decisions; it delayed hearings; it delayed denials; it delayed payments; it delayed everything.
There are other reasons, but these are the main reasons.