There were some older games in the Top 100 (25 from the 1990s and one from 1985), but I think the reason so many of the games were recent releases is that the majority of the people who ranked the games are relatively young. Now, if you had a bunch of fossils like myself (ahem...cough, cough...) creating this list, it would be different and include many more older games. But if you get a bunch of 20-something or even 30-something reviewers who haven't experienced many of the older games, you'll end up with a list such as this, biased toward more recent games. It's all opinion anyway and this magazine puts out this list just to stir up debate and to perhaps draw attention to some of the reviewers' personal favorites, which isn't necessarily a bad thing. Just take every list of this sort with a grain o' salt... I imagine if PC Gamer's readers voted, the Top 100 of All-Time would be much different.
With that said, here's PC Gamer's rationale regarding why they chose the games on the list:
"The Top 100 simply represents the games we love, ranked by how much we love them. Our goal is to give you an actionable resource--a list of what you and any PC gamer should play today. To that end, historical significance doesn't matter in our selection; weighing nostalgia over what's fun now wouldn't provide a useful list."