I'm running off memory here, but if I recall correctly, Abrams showed strong numbers with Independents. Since I'm a big proponent of Bitcofer's negative partisanship theory, I put a lot of weight behind carrying independents.
Forgive me if this repetition but a quick oversimplified version of the Bitcofer theory: Democrats and Republicans are reliably going to vote for their party. Look at Trump's approval rating for proof of concept. Independents don't actually swing back and forth either, but which ones actually show up to vote can vary. So you want candidates that can either turn out independent voters that lean their way (moderates in this case) or you need someone on the opposite ticket that galvanizes the independents to show up just to vote against them (negative partisanship is the term for this).
So what I want, is a candidate that did numbers with Independent turnout to combine with the negative partisanship effect of Trump being on the opposite ticket. Abrams is moderate enough not to galvanize the opposite side's Independents while also having some poll data to support the theory that she would turn out independents...and also, the other moderates that make it into the discussion are usually people I really don't like.