Apple II nostalgia is not due to modern Apple's success. The people keeping it alive actually are often hostile to modern Apple, never forgiving them for killing off the Apple II in lieu of the macintosh. (See "Apple II forever" or attend a Kansasfest sometime)
a lot of key early gaming milestones happened on the Apple II, for a variety of reasons. One was the extremely clever floppy drive controller made by Woz. You could have actual decent sized games that would load in seconds. I know there's a lot of weird nostalgia for waiting large fractions of an hour for screeching bits to load off of tape, but that wasn't great for complex games.
Don't overlook the education market, a large set of Gen-X used apple IIs at school, and there was an immense educational software market
Business was big too, especially Visi-calc. It was used in much more serious ways than later 8-bit machines. To the point if you look closely at the original IBM PC you can see its design closely matches the Apple II in many ways, as that's what IBM was worried about. The c64 didn't even exist yet when that was happening.
Expansion, having a machine with a case with room for 8 expansion boards of fully documented slots is so much fun for hacking compared to trying to shoe-horn things into a mass of wires and loose-connector cartridges hanging out the back of your machine.
Finally cost, the II has a reputation of a rich-kid machine. Which is true, but everyone gets all excited about DOS gaming and during the DOS/PC age most PCs were really expensive as well.