I don't think that's just it. In part it's nostalgia. In part it's what @
Temp name says, the limitations of 8bit forced composers to be more creative and catchy. In part is what @
XDurionX says, as there are hundreds or thousands of 8bit music that "no one" remembers, so we are comparing average of today to the very best of old days.
It's the same with movies for instance, we tend to remember music from iconic movies and tv-series, not only because they might be good music, but because of what that music is associated with. For instance the Walking Dead intro theme - you'll probably never listen to it as "music", but if you watched it it will likely feel iconic for the rest of your life.
And it's also the same with music in general really, we remember the classics and greatest hits, and are quick to judge how most modern music is crap - because we simply forgot about all the crappy/forgettable music that was made back then as well, and only remember the good ones well.
Another thing is it's just a process of growing older. Not just on a neurology level, but on a mathematical level. The more you experience, the more you establish your taste and a base of "knowledge" that you will compare everything new to, so the older you get, the harder it is to find new things you really like on that sort of level, especially things that aren't somewhat like what you already like.