Wouldn't they first see if he is a popular guest appearance guy in flash/green arrow/supergirl first?
This is a pretty obscure character.
Heck, when he was in the Superfriends in the late 70s they weren't even allowed to use his name for some reason and called him Black Vulcan.
I wouldnt call Black Lightning obscure more than I would, say, Wonder Twins.. And look at how much their first appearance is going for.
TV show or not, this character is well known among people my age, I loved BL as a kid, more than I did Black Panther or Luke Cage, which I think were the only 3 major black characters on the DC/Marvel universe... Even Falcon didnt have his own title in the 70s.
Why did Black Lightning not receive as much attention is beyond me