By waiting until 2016, Ms. Clinton may have let her moment pass her by. That moment, with the advantage of hindsight, was most likely 2004, when her husband's presidency was recent enough that it was defined more by the strong economy of the 1990s, not by the seeds of the 2008 economic collapse (as many on the left see it now).
Moreover, Ms. Clinton was, while still well known and familiar, was not quite as much of an establishment figure as she is today. Additionally, the 2004 Democratic field was not a strong one and consisted entirely of white men from the political class. In that field, Ms. Clinton could have presented herself both as an outsider because of her gender, something that she was not able to do against Mr. Obama in 2008 or thus far against Mr. Sanders this year, and the heir of the then-untarnished Clinton legacy. Moreover, in that truly multi-candidate race, it would not have taken much for Ms. Clinton to win a plurality or come in second in all the early states.