My beliefes:
Part demand. - A lot of people around the world dig these systems, and it has more of a following then say some of the more obscure systems, yet it wasn't the best seller in it's heyday compared to others, people love an underdog. - Also those 20-30 somethings who remember the system, who don't have families yet and have larger incomes of extra money are at the perfect point to raise demand.
Part Just not as many were made in a lot cases compared to SEGA - NINTENDO - SONY
Part Rareity in some cases.
Part Video games and collecting becoming more of a main stream hobby for many, and people are simply willing to spend more on them.
Part Internet - increased ability to see what games are selling for and get good deals has increased prices for everyone as demand increases. It has also made it easy for stores to price something at $x becuase it sold for $X once and it becomes it worth them sitting on something to make $x profit.
To answer your question. BOTH