The lack of field organization to begin with.
The lack of even remotely sniffing the three early states.
And the simplest fact...it profits her nothing to run a race that will split at least three ways, and that the MSM will go more than the distance to protect the incumbent.
The only question in my mind is when/whether she makes the decision that the price her family pays for her running is worth the price America pays if she doesn't run.