Portal is really just something different that I don't think should be compared.
Portal is just a dumbed down version of Narbacular Drop. What is it you don't think it should be compared to? Because it wasn't a Half-life mod, it's not indie? It's really just a matter of timing, right quality, right content, right team, right time.
Sure. But that's true regardless of whether you're writing a mod (CounterStrike) or a game from scratch (Portal).