There are limitations on who can produce licensed figures at certain scales and where they can be sold, it's the same reason why you don't see NECA's TMNT figures on the pegs at Toys R Us.
Mattel has the license for most of DC's mass market 6" stuff - they likely got a little antsy after NECA released some figures centered around real-life people (Michael Keaton Batman, Superman vs Ali two-pack), and seeing NECA priming to release full-blown comic based stuff they probably said "Enough is enough" and got the lawyers involved.
There's some other examples of licensing things like that - all Funko POPs for "Star Wars" and "Marvel" are bobble-heads, Marvel Select is deliberately out of scale with Marvel Legends (and aren't sold in big-box retail stores, and have a limitation on the number of points of articulation they can have compared to Marvel Legends)