I'll read the other replies after making my own.
1. Aesthetics. Many of the old patterns just really "do it" for me. The trapper, the swayback jack, the eureka, etc. Then there's the handle materials. Wood, bone, stag, etc. Yes, you can get these on modern knives, but that doesn't stop the traditionals from looking good, or even better, with those materials.
2. Simplicity. I can get my 1095 GEC sharper easier than others can their S90V and Rex 121 blades with hundreds of dollars worth of diamond stones.
3. Practicality. Most slipjoints have nicely thin blades that just cut well. Better than most modern folders, definitely better than most "tactical", and far better than the "tacticool" nonsense (at least, I think it's nonsense). I don't need to pry open a door or baton wood or hack through a car roof in my daily knife, so a blade that's thin like an Opinel or SAK suits me far better than something like a Medford or Strider. And I don't need a lock that support the weight of a waterbuffalo hanging off it either.
4. Versatility. Granted, this doesn't apply to single blade knives, but something like a stockman, congress, SAK, wharncliffe trapper, etc, gives you multiple styles of blade in a small package. I like having at least one curved and one straight myself.
5. Honesty. I feel like a jackass whipping out a large knife to cut something that doesn't need it. Heck, the largest folding knife I carry is a Bareknuckle, and it's only 3.5", yet feels huge compared to my Delicas, Leeks, etc. I'm not going to bust out my Bareknuckle, launch it open with the flipper in a flashy, noisy way, just to...you know...open my mail or trim a loose thread. Not with other people around, at least. Which brings us to...
6. Yes, the sheeple. Even in Arizona where I can carry whatever knife I want, and people openly (and concealed) carry firearms, it's still my preference not to whip out a modern knife (which so many people, right or wrong, recognize as a weapon rather than a tool) in front of mixed, unfamiliar people. Something small, and something that resembles what peoples' grandfathers carried, or a SAK like MacGyver used, doesn't startle people and make them think you're someone to worry about.
7. I just like them.