If you want a very simple explanation why Hillary Clinton is not only the 2016 Democratic frontrunner but also able to scare off any serious competition, you need only look at this graph from Gallup.
Regardless how you feel about Clinton, this is remarkable. This level of familiarity among the public is unheard of for anyone who is not the president. Just look at Joe Biden’s numbers by comparison. He was a senator for decades, ran for president twice and has served as Vice President for the past five years, yet his familiarity number is still 11 points lower than Clinton’s. Paul Ryan who was part of a national presidential campaign less than two years ago doesn’t even come close.
Hillary Clinton is not an incumbent president but she effectively has the numbers of one.