Well, the issue, I think, is that your defenses are supposed to go up mainly based on equipment. BAB goes up, and hit points go up, but the classic D&D paradigm, your defense bonus does not. D&D 3e has changed that to some extent, but there are still remnants of the old model. Thus, lightly armored fighters who are viable are bucking the trend.
For comparison, fencers under AD&D 2e who took the correct kits could expect a -1 AC. One point. They could garner another point by taking an appropriate style proficiency. That was basically as good as it got. Monks? Started at AC 8 or so, the same as Joe Average in leather armor, or most dangerous animals, and slowly improved if you managed to survive to mid levels, and bracers did not stack with their AC improvement. IIRC, the original version didn't even get their Dex bonus to their monk AC, despite a respectable Dex requirement just to take the class in the first place.
Under basic D&D, your defense was your armor, and your Dex bonus, and that's it. And improving your Dex beyond 15 (+1 bonus) took an insane number of wishes.