I think it's because people are trying to find themselves in the games they play. back when HL1 came out, video games were considered a very nerdy activity (most people were still thinking of the stereotipical girlfriend-less nerd), so, exchanging the man of action from previous games, with a guy in a labcoat, made it for the players to identify with Gordon.
When HL2 came out, the old players contiued to identify with him, but as games had a broader span than before, so a lot of new players join in, some because they actually identified with Gordon, and some because it was a trend to identify with him and some actually did not even care about him/hated him.
In today's games, game companies are trying to make the characters be loved, but in essence (especially in shooters) the man-of-action is still dominant, as games have gotten an even broader span. Today, if you make a door-to-door salesman as a main character, you're not going to get the same response as Gordon Freeman had when HL1 was released, because most door-to-door salesmen don't play games (I might be talking out of my ass here, but I think that most salesmen don't play games).