I suspect the majority of the 60s tv audience was familiar neither with the GH radio show nor the Batman comics. They were first and foremost TV watchers (just as MCU watchers are mostly not comics fans).
On the contrary. The Green Hornet radio series ran from 1936 to 1952 -- a sixteen-year run that ended less than 14 years before the TV series premiered. It was aimed at young teenage audiences, and it was one of the most popular radio series of its day. So if we assume its audience was mainly 13- to 16-year-olds, they would've been in their late 20s to mid-40s when the TV show premiered -- a substantial overlap with the prime 18-to-35 demographic that TV advertisers usually target.
So we're talking about something that was a hit roughly 15-30 years in its audience's past, which is the equivalent of something like, say, Batman: The Animated Series or The X-Files from the perspective of a present-day audience. Would you call that obscure and forgotten? Presumably they remade The Green Hornet because it was popular and remembered among the generation that had grown up to become the creators of new programming, the same as it is with remakes and reboots today.