I've explained twice now why that in fact wasn't
a totally wrong idea, it was the truth. There was no point making any bigger deal out of powerful threatening races than just having Q mention them, if said races had no reason to be a threat relevant to the Federation heroes that the show focuses on. It makes sense
that these threats aren't mentioned again until their home turf is revisited by Voyager.
I also don't see why you're pointing a finger at Q Who
for not having a whole backstory prepared way back then, when they weren't even expecting a recurring role. Cardassians weren't well defined until DS9, Andorians weren't until ENT, and so on. The responsibility for fleshing out these and the Borg fell on the creators of DS9, ENT and VOY respectively, because they were the ones who chose to make them recurring. Not TNG and TOS.