Because, what other shooters tried, half-life accomplished.
- Half-life had weapons that actually felt specialized, and different, not just bullet spamming machines.
- Half-life, by never letting you away from the first person perspective, made you the character, rather than simply letting you play as one.
- Because of the hours of QA and play-testing that went into Half-life, the game managed to include puzzles, obstacles, and difficult enemies without ever slowing the pace down.
- Half-life had a "real" story. That is to say, one that was plausible, and a bit scary.
I think the big thing about Half-life for most people wasn't that it shone in any particular area, but that it shone in just about all of them, and was polished until it shone. Valve was the original "you'll get it when it is done" company.