Often times, older games seem harder to us because when we were younger we'd play them, get frustrated because we got lost(Original Metroid anyone?), stuck somewhere and couldn't figure out what to do, or just plain got game over's all the time. So our minds throw the idea of "This is impossible who actually finds this easy?" When really, the games weren't too hard(not all of them anyway, I'm looking at you 007 Goldeneye N64 <-<), we just didn't get it like we do now. I'm not saying there's no "Nintendo Hard" games, those exists and live to anger us gamers, but usually when you go back and play them again they don't seem as bad.
It's partially technology gap, but also the fact that as gamers we remember a game's difficulty by our own experience with it. And it's not always a cake walk experience either, so yeah curse you Goldeneye N64 and Ocarina of Time water temple! ...I mean, yes older games do seem more challenging that newer ones, but blame lazy game dev's and uncreative ideas for most of that.