Some of the armies out there are unlikely to ever see in the starter box. Let's look at twelveyear old Little Timmy and his dad. They've never heard of Games Workshop before today. They come into their local GW store and Little Timmy, who is a big Science Fiction fan, wants to know "Who are the 'Good Guys' and who are the 'Bad Guys'?" Now, with Space Marines the simple answer for the newbie like Little Timmy is, "The Space Marines are the 'Good Guys'; kind of like intergalactic knights." But what about the Tau and the Eldar? If they were the opponents in the starter box to the "Good Guy" Space Marines, that would make them the "Bad Guys" in Little Timmy's mind. Little Timmy won't understand that there are no real "Good Guys". Many would argue that the Tau and Eldar aren't evil (they don't look evil), so Little Timmy gets confused. He bigins to lose interest. So, while the Dark Eldar are always an option for a starter box, their "Good Guy" cousins are too good for their own good so to speak. Same as for the Tau.
What about Daemons? Well remember, Little Timmy's dad is going to be footing the bill for this game. He's ok with Orks ("Goofy green fantasy-like monsters that sound like something out of the Lord of the Rings") or Chaos Marines ("Those Space Marines are green and these are red, sure, whatever") or Tyranids ("Monster bugs, kind of cool, like in the movie Aliens"), but Daemons are just too evil ("Demons? Like from Hell? Let's go Little Timmy. I'll buy you a new X-Box One game"). As far as Necrons, they are a difficult choice for a first army. I mean they are cool and all, but cold and impersonal for a new player with which to identify.