Good point. But what makes you think Warren isn't interested even if Hillary drops out? I'm not necessarily disagreeing with you, but it's possible she just doesn't want to enter as a massive underdog against Hillary.
Klobuchar, for example, met with the Iowa and South Carolina delegations at the 2012 DNC. She took time out from her own reelection efforts to campaign for Christie Vilsack in Iowa. She attended an Iowa-hosted party at the 2013 Obama inauguration, and made several visits to Iowa in 2013. She raised money for Shaheen in NH. She gave several "non-denial denial" answers when people asked her if she was going to run for president back in 2013. Her ambitions are of course now on the back burner, since it looks like Clinton is going to run, but she made her ambitions pretty clear.
Warren did nothing comparable, even going back to late 2012, when it was less certain that Clinton was going to run. She answered "No no no no no", when asked if she had presidential ambitions back in 2012, and spent much of her first year in the Senate avoiding interviews and trying to downplay her public profile.
