In early editions, monsters required careful planning because of the relative fragility of both monsters and players. When you have three hit points and a sword does from 1-6 (and it hits roughly half the time), you are not taking any chances with swords or arrows. Combats tended to revolve around surprise, traps, and ambushes. You snuck up on them and got your bonus round, or else you made sure there was more of you then of them. In this, fighting looked a lot like real life- as damned unfair as either side could get it.
Monster design was necessarily pretty brief, because you weren't really fighting them for long. You could describe them in a single line, because the only part of the game the rules really touched was the fighting, and the fighting didn't last too long. Or occur too often. You bashed in a skull or two, licked your wounds, and tried not to encounter too many wandering monsters, because, again, any given encounter could spell death. (This was the earliest, and probably most effective, deterrent of the famous "fifteen minute adventuring day", by the way.)
|Pictured: a daring ambush by two skilled adventurers|
But as the editions progressed, as this chart handily shows, it quickly becomes less necessary to skulk and hide and fight. You're a hero, suddenly, and you're able to single-handedly beat the crap out of seven goblins. And you notice that, at nearly exactly the same time, killing monsters becomes the focus of how you gain experience. Since players are going to be gravitating towards the things that give them the most reward (and wouldn't you?) suddenly you have a game that revolves around killing monsters, with their stuff being the reward for killing them, instead of being the main goal.