Lol what? Wall of Light was created in Dark Empire, though it wasn't named this at the time
Maybe because they don't make good stories...
"Oh my god, Exar Kun is unbeatable. What should we do?"
"Wall of Light"
"You know, that random made up power we just invented. Totally not deus ex machina..."
, 'Band of Light' was used by the spirits of all the Jedi to stop Palpatine from returning again, it was also used in Tales of the Jedi before it was to trap Kun, when Odan-Urr used it to cut off Massassi Warriors from Naga Sadow and was also used in it's lesser variant 'Sever Force' by many before it was used on Kun.
Your statement is flawed, it was not invented to defeat Kun.