The people who grew up with these games are now old enough to buy them, and so the prices will go up. Right? Also lack of supply and all that.
Also, yeah, I don't have any solid statistics to point to or anything, but it seems like electronic video games have increased dramatically in popularity since I was a kid.
I firmly believe that demand continues to increase, for a variety of potential reasons including (but not limited to):
-- Nostalgia for consumers who grew up with these games & would like to play them again
-- Increase in consumers due to retro popularity which has been fueled by web series such as AVGN, The Game Chasers, Pat the NES Punk, and more
-- Some consumers who have rejected Gen8 consoles turning to older consoles for gaming
As more people buy and collect older games and systems-- especially those with significant nostalgic value (Mario games, Mega Man games, Contra, et al) or those limited in availability-- prices inflate as a result of higher demand. More people want these games, their perceived values increase, and we all pay a bit more.
Demand is only part of the equation. People selling these games are understanding what they're selling a bit more. eBay and Amazon pricing is used. Games are being priced individually more often than being placed in lots. Even thrift stores are learning that retro games are a hot commodity, and the more obvious donations are being priced higher.
Finally, I do after that video games are more popular now than ever before. They're certainly a more recognizable part of culture for wider audiences than they were in the 8/16-bit eras. That can swell the pool of potential retro buyers.
EDIT:
I wish I can afford retro collecting. It's my dream to one day be able to buy all the retro games I crave, create some sort of video game museum in my home.
The oldest games I currently own are N64.
The good news is that it's not ridiculously expensive to build a retro library, especially once you get past buying the hardware. There are still plenty of common titles out there which sell for less than $10 apiece, and many of these run for <= $5. It's the "big ones" that you have to budget for or get lucky in finding at thrifts or flea markets; JRPGs, Mario games, flagship NES games, etc.
It takes a bit more patience to collect without overpaying these days, but it can be done. Deciding to not care about complete games and being happy with disc/cart only has saved me a fortune in the 2+ years I've been building my library.
And I'm sure that the GAF Retro Collective here can offer even more tips, if you're looking to start a collection.