And for strength it only started at 15 too. Same for con. My eperience seems to differ greatly from yours.
Charisma did not play a big role at our table. So maybe it really depended on how the DM made use ofndifferent abilities...
Yeah I've played AD&D for years, and even the best DM I know never actually rolls for NPC reactions. He sort of eyeballs the player's Charisma and Comeliness*, largely ignoring the systems involved and decides how an NPC reacts.
*Yes, seriously.
As for Str vs. Dex in AD&D, Strength wins. Accuracy and damage bonuses are worth their weight in gold*. Oh sure, you can get a small bonus to ranged attacks with Dex, and a 5-20% AC adjustment isn't nothing, but armor is far superior (though I won't argue armor + dex is nice).
*I mean think about it, how do you get bonuses to hit in AD&D? Ability scores, magic weapons. How do you get AC bonuses? Magic armor, magic shields, Rings and Cloaks of Protection, Bracers of Armor, Ioun Stones, Cloaks of Displacement, Robes of the Archmagi, Boots of Speed, et. al.). On a long enough timeline, you can have a guy with AC -10 wearing nothing but a barbarian loincloth!
The issue here is how AD&D handles high ability scores. For most players, even with a 16, the benefits can be marginal, and the real benefit is possibly getting the 10% xp boost for your class. It's difficult to talk about ability scores until you're looking at 17-18 because of how massive the benefits can be, thanks to Gary's "lightning bonus round" approach to ability scores.
For a Thief, a 17 Dexterity is like gaining a free Thief level! For a Fighter, a 17 Con is 9 more hit points over the course of their career. And of course, someone rocking an 18*91 Strength is a best in combat, likely one shotting Orcs like they're nothing.
So for any melee character, you want Strength. You want lots of Strength. Because the benefits are explosive and immediately noticeable. Strength can also let you wear heavy armor, carry lots of treasure, perform heroic "impossible feats" and even bust down magically locked doors if you have enough of it!