nostalgia...that's the only reason, it takes us back to our youth, and youth one loved but can not go back to, playing the old game just brings back those happy memories and that's it.
I for example can not play FF7 or Zelda OoT, they look and plays like shit. I do not have any emotional attachment to those game's since I never played them in my youth so to me these games are just crap and now matter how good the quality the games were back in the day they don't stand the test of time, but their are people who loved them back in the day, and have fond memories, so for them they can go back and play them despite their flaws...nostalgia is very powerful, and publishers know how to milk it, which is why we see certain old games do well.
The one game I loved back on the day was Revolution X for the sega mega drive, basically a light gun type based on aerosmith game but me and my bro used the standard controller for it, and back them it was awesome, but if you go back know....it's a piece of shit, but you know, because I have fond memories of it, I overlook the fact it's a piece of shit and some what enjoy it as I did back in the day... so to say "some old games prove to be of better quality that some games today" is simply ludicrous.