That's because they've never been presented with the opportunity until WB decided to test the waters at Six Flags with the Suicide Squad scarezone/meet-n-greet.
Due to the popularity and success of that scarezone, it returned last year. And while the 4 DC characters featured (Joker, Harley, Katana & Killer Croc) were merely just photo-ops, the casting still went through WB and was very well done. As for the scares? They had a few molten black zombies roaming the area. There of course was wrecked scenic pieces throughout the DC Universe as well. Could it have been better? Of course. But for venturing into new TERRORtory of featuring DC at a haunt event, it was enjoyable.
Arkham Asylum has always been a maze fans have been begging to have at HHN back in the days of The Vault. So its kinda cool to see what WB does. Honestly, I could see it happening at Six Flags one year too. But we'll see..
Except for the fact that when he was featured at Six Flags for Suicide Squad, he was a costumed character
Thats the thing with DC... they have a multitude of iterations to choose from... comics, cartoons, videogames, and live-action films.. So it will depend on which direction they want to take.