I disagree. Many VP picks have been people that weren't widely known outside their own state when selected as a running mate. Abrams is as well known, if not more so, than Val Demings. Frankly, I'd never heard of Demings until waroche mentioned her. I had to look her up. But I knew who Abrams was. This is largely due to all the national attention to the last Georgia governorship election. Abrams' record fighting gerrymandering and voter suppression in Georgia is impressive.
As for why she isn't running for a Senate seat, you'd have to ask her.
To claim that Abrams can't help win the elections the way 'Warren, Harris or others could' is just an opinion and not one agreed with by many. While I like Warren and think she would make a great VP, she is also needed in the Senate. Additionally, MA has a Rep. governor meaning that her Senate seat could be filled by a Republican. Since MA law requires a special election 140 to 160 days to elect a new senator, that means the appointed Republican senator would be in the Senate for several months. If the Dems are successful in narrowing the GOP Senate majority, one vote could be very important.
I just don't see Harris being the pick.