Wait... 9 players for 1e, 5-7 for 2e, 4 in 5e... Are D&D players losing friends over the decades?!
I suppose if many people started playing as kids in the earlier versions and are now adults then that would check out
The most common reason for the lower number of players is because it's more fun in TT when you get your turns more frequently.
Sitting around waiting for 8 other people to take turns before you can do something again takes its toll. (This also bleeds over into video games too, as the more party members the game is balanced around, the more enemies get put in and with the typical TB format, this means you can have to sit around for 10-20 minutes each turn waiting for the 20-30 enemies to all take turns...)
In video games, the lower the party number the more "Replayability" since if you can do literally everything and see every interaction in 1 playthrough, there's less reason to play the game more than once.
