Because the Rogue does not get to say "This pit trap has a switch over here that turns it off" and the Wizard doesn't get to say "Magic Missile is the obvious counter to his Frost Ray!" -- those choices are outside the player's control.
If the rogue rolls well enough then they do, at least in 3.X, get to say that there is some way of turning the trap on and off (i.e. disabling it in such a way that they can enable it again later). The spell "Dispel Magic" can dispel all sorts of spells - how does the wizard know what precise magical effect will work against that specific enchantment? Simple - they get to specify that there is a weak spot in the enchantment that means it can be dispelled.
Combatants have always had the ability to use motives and goals against the opponents. Discerning players have always used moves against the expected motives of their opponents.
On a gross scale, of course - knowing that the enemies are protecting a specific thing or trying to get a specific object is something that can be leveraged. But there are all sorts of tiny-scale motives and intentions that are revealed in body language, positioning and other "tells", that a skilled fighter can take advantage of. These minute clues and instantaneous motives are not detailed in any roleplaying system that I have experienced, mostly, I suspect, because doing so would take forever. It's an area where abstraction is almost mandatory - but the more common course is simply to ignore it altogether, crippling the "mundane" character types in the process.
The Fighter's player gets the ability to assign those goals and motives in order to correspond to the abilities used. "The dragon becomes overconfident", "The wizard tries to escape past the Fighter", "The assassin falls for the feint" all assign motives and actions to characters outside the normal control of the player and regardless of the psyche and behaviour set of the opponent.
Maybe I wasn't totally clear; the fighter
doesn't get to "assign" the motives and goals at all - the fighter gets to determine
that there were some motives and/or goals that s/he could use. Given that such motives and goals occur all the time in any opponent who is not wary specifically that they may be used against them, it seems to me to be an entirely reasonable supposition to simply assume that there are some such motives and goals present.
I've been thrown twice in a row -- using different techniques, but with the same result. I have no doubt my opponent could have tossed me a couple of more times as well if he hadn't decided to ease up.
Different powers could render a fighter prone just as well; are you saying that you weren't more wary of similar moves after you had been thrown once? Perhaps, even, that was why your opponent used a different technique the second time?