Four years is a long time in politics. Very few people outside of Illinois knew of Barack Obama before he addressed the DNC in 2004.
And to be fair, although I like her a lot, a fair amount of her popularity is because she hasn't had to deal with the dirtiness and ugliness of a presidential campaign. I guarantee it would be a different story from the minute she announced. If she's got an exploratory committee going, it's very much on the dl.