One of the nice things about modern, curated compilations of games from old platforms is that they provide an opportunity for “lost” classics to finally get an audience.
In many cases, “lost” classics were completed and reached a full prototype phase, but just never ended up getting duplicated and distributed to the public. Sometimes this is understandable; at other times, it’s a bit of a mystery.
Aquaventure for Atari 2600, which you can play not only in Atari Flashback Classics but also as part of the Atari Collection 1 cartridge on Evercade, definitely falls into the latter category. This game is good, so why didn’t it get released?