A couple of reasons actually. First, and most obviously, they're newer and thus haven't had time to start to decay yet. Secondly, publishers tend to use a higher quality paper than they used to. The higher quality comes from less acid being used in the paper's production. Of course, this comes with a higher price tag, which is part of where many of the price hikes that really began in the 1980s come from.
Third, and perhaps least obvious, is that the paper the comics are printed on are given a coating. When comics used to be printed on cheap newsprint, the ink was placed right on the paper. The paper then absorbed some of the ink, and it would spread through the porous material. So even new comics back then looked a little fuzzy, since the ink would bleed away from where it was initially put. In more recent years, the paper has been given a thin non-porous coating so that the ink technically never touches the paper; it sits on top of this coating just above the paper. Because the coating is non-porous, the ink doesn't have anything to bleed into and looks more crisp. So we can see more detail and get brighter colors.
The coating that's used is generally a substance called kaolin. The name is derived from Kao-ling, a village in China where the substance was found in abundance. Because of this location, that's why it's also sometimes called China Clay.
Yes, clay. As in, the same type of stuff you made that ashtray out of in grade school. It's a slightly different type of clay, and typical has a whiter appearance, which adds to the crisp look of the paper that it's coating.
But it's still clay, and still smells like clay. So, now, if you open a long box of mostly newer comics, it smells more like a pottery than a library.