How so?
I'm going to use an example : the rocket launcher turret thingies. There were like.. two in the whole game? There could have been potentially tons of puzzle using them (of course, making the game too long would have been a mistake too, but Portal could have easily lasted at least one hour or two more without hurting the game IMHO).
I don't remember any rocket launcher turrets in the campaign. Padding it out with a bunch more puzzles would have taken away from short, focused style they were going for.
Also, it was released as a $20 game.