When I first started playing AD&D back in the early 80s, it wasn't uncommon for players to run two PCs apiece, especially if there were only two or three players (besides the DM.)
Heck, it wasn't uncommon for the DM to throw his own PC into the mix, especially in the "shared world" context of "there was no world besides dungeons anyway." DM PCs were handled by the group at large, and never we allowed to search for traps or find secret doors - just too bloody convenient for the DM if they could bypass the traps and find every secret door.
So, when did we turn away from multiple PCs and henchmen (who were always ready to step up as a new PC if their master took a dirt nap?)
You could argue with the power creep to the classes with each edition, there was less and less need for additional characters in the average party, but that doesn't explain why those playing retroclones tend not to play multiple characters and have few if any henchmen and hirelings in their groups.
Another question bouncing around my head thanks to Save or Die Episode 99.