We don't talk about Pun-Pun
Giving this more thought, I think I know what happened. Some of the DM's our group has had over the years took all the wrong lessons from the 1e DMG and believe every fight should be a meatgrinder, magic users have to be kept down at all costs, and a worthy reward for an epic dungeon crawl is a dented copper piece and a potion of delusion (healing) (I'm only being somewhat hyperbolic).Mine's 11th, approaching 12th.
Highest level characters in this side of that campaign (it's dual-branched) are 13th; about the same number of xp as my MU, just in faster-advancing classes. Highest-level anywhere in the campaign just got to 14th, that being a War Cleric who's been in play intermittently since 1981.
Highest-level MUs I've ever DMed were 10th; in campaigns where the highest level anything got to was 12th.
Long-term over all the great many characters played in our games, it's been pretty close to 40-20-20-20 F-C-T-M, ignoring a very small smattering of Monks and Bards.
So it's no wonder that warrior classes became popular, as they deal the most damage at low levels with the highest chance of survival, lol. No explanation for all the Thieves though; in my opinion, playing a single-classed Thief is almost as difficult as playing a Wizard. Only high stats and pure optimization can get you slightly better at a coin flip in most Thieving abilities at low levels, the penalty for failure is almost always certain doom, and most Thieving ability use typically suffers from the most painful reading of the rules, lol.