Main reason being fundraising and support of the establishment. Obama was an exception in 2008 because of the extreme nature of the situation - democrats were feeling hopeless after losing in 2004, and even the frontrunner (Clinton) had never really run a national campaign before.
This time, I think, if Clinton runs, she's almost a shoo-in. If Clinton (or another democrat) wins, he'll serve a full term in the senate, and run for President in his second term (2024) with some sort of a record. If the Republicans win, he'll run at the end of his first term (in 2020). It takes time to build up a fundraising and support base.
I think the Democratic candidate that's a little 'under the radar' is Andrew Cuomo. But he's very close to the Clinton family, so if Hillary Clinton runs, he may not run out of deference.