You can spend all day reading and listening to why she lost but in my mind the answer is simple. There was no buzz about her, no excitement. In 2008 there was buzz about Obama and in 2016 there was buzz about Trump. When elections are this close that only a few thousand votes make the difference, buzz counts. Debate instead about why there was no buzz and you'll find out more.
Side note: I see where Biden says he'll run in 2020 when he'll be 78, HRC will be 73 as will Trump. The leaders of the DNC are all in their 70's now as am I but I suspect by 2020 the country will be ready for some young blood and if someone comes along that can generate some buzz they'll be the next president.