Coming into this case only since March I’ve wondered the same. I think it’s because she is known to have had a rather extensive social media presence. I don’t know that she was particularly active on all the sites, but she did have quite a few accounts (FB, more than one twitter, instagram, kik, tic toc, etc). I don’t believe Abby was as much a part of the social media scene. Granted, this seems to be rationale for the crime being targeted, and having something to do with social media (which I personally do for the time being).
That’s the only reason I can think of, but perhaps there are others, and I’d be interested to hear.