Maybe that is one of the reasons she has waited to announce. Gives less time for challenger to prepare. Zero is very weak candidate.
I believe one her chief reasons for delaying her announcement this long, is to allow the statute of limitations to run out on the frivolous lawsuits from her time as Governor.
That said, I also don't believe that she wanted to be a part of the pre-primary demolition derby we're watching now. Sarah's got about the sharpest political instincts that anyone's seen in a lifetime. You can bet that whatever her strategy is, it's been engineered for maximum impact and effectiveness.
Read Sun Tzu, and see if you don't see a lot of what Sarah's been doing over the last year, staring back at you.