If Age of Sigmar has been "successful" in GW's eyes, they might be making a reasonable assumption that people are willing to spend $200 on a game, where they wouldn't spend $1000 on an army for WHFB... for example.
Something I loved about Blood Bowl was that I could buy a starter "army" for $50 [in the before times] and then spend another $50 to buy the rest of my "army" to fill out a roster. I could buy an paint an "army" in a weekend, if I was really in a mood to do so. I mean, it always took me longer than that. But I could
have. I still have a Human, Orc, Dark Elf, Dwarf, and Chaos team kicking around.
So if they're finding that people are more willing to spend $200 [several times] they might be inclined to restart the Specialist Range. If their current M.O. is to soak someone for a Starter Box, then maybe sell a couple hundred bucks more goodies before people ditch, then the specialist range would certainly fit that bill. I suspect they ditched Specialist Games, because they were under the impression that everyone loved their models soooo much they all wanted to buy $1000's of models. But that doesn't seem to be the case, it gets too intimidating. So the specialist range lets them sell you a $200 "army" and then if you like it you buy another $200 "army". There's less "Oh amphetamine parrot, I've got to paint 200 Clan Rats before I have a painted army..." You can paint 15 duders in a weekend, and you can play Blood Bowl forever.
As a gamer first, painting can kiss-me-where-I-poop-type hobbyist, the Specialist Range was perfect for me.