michaelleung: Because that's all my computer can play.
That's definitely part of it for me. My laptop was on the more potent side when I bought it in 2006, but nowadays it's not up to snuff for many of today's games.
Nostalgia plays another part, and the opportunity to play games that I simply didn't have the opportunity to buy when new. Master of Orion is an example of this, a game that I really liked on the couple times I tried it on a friend's PC but didn't have the wherewithal to buy it myself at the time. By the time I could buy any game I wanted, it was no longer around.
And honestly, price plays a part. I can spend $60 on a modern-day game for a 10-hour play-through where the rest of any fun comes from online play or reliance on the mod community to change the solo game, or I can spend $60 to get 6+ games that I can play solo for many more hours. From a value standpoint, old games have some significant advantages.