Microsoft releases a version of an operating system. Then they work at it making it better and release an update. And so on. Microsoft is still selling Windows, but Windows today is not the same Windows from 1990.
Some board games also evolve internally, or are adapted and developed by others, in the same way that features from operating systems, browsers, and word processors develop in-house or by building on the competition. Games move online, mechanics from other games make their way into existing ones (like the latest versions of Risk), or other companies make competing but better products.
So why did Hasbro update Risk, but leave Monopoly's rules alone? Why are we still playing the same Chess, Scrabble, and Battleship, when, with rare exception due to compatibility, no one uses computer applications that are older than ten years? Sure, evolution leads to a numerous dead-ends for every viable step forward, but nonetheless there are steps forward.
I don't think it's just bad marketing. I don't think it's just a monopoly on the toy shelf.
Many games are bought for other than entertainment purposes: ritual games (like Snakes and Ladders and Mancala), keepsake games (like Monopoly version XYZ), pretty games (like Chess sets).
Also, there is a perception that "new" games are only niche products. Yet, 38 of the top 100 games at Amazon are "new" games, counting various Scene-It's, Cranium's, and Killer Bunnies expansions (and Settlers of Catan and expansion, Race for the Galaxy, Blokus, Puerto Rico, Power Grid, Dominion and Dominion Intrigue, Lost Cities, Carcassonne and expansion, Wits and Wagers, and others).
Of the remaining games, many of them are new licenses for older games (like Spongebob Squarepants Operation, or Beatles Monopoly); a licensed game is bought for its toy value, not its game value. They don't really count.
New games are likewise 32 of the top 100 board games at Target (more party, less strategy games), 26 of the top 100 "games and puzzles" at Walmart, and 24 of the top 100 "games and puzzles" at Kmart. On the other hand, they're only 7 of the top 49 at ToysRUs.com, and 2 (if you push it) of the top 24 at boardgames.com. I think these guys simply don't stock many newer games.
Thirdly, there is a perception that people only play board games to teach kids a social activity that parents or grandparents played when they were younger (doesn't everyone play video games, now?)
That doesn't hold true for music, mostly. It certainly doesn't hold true for video games, though there is a small reactionary movement in that direction. It rarely works for movies, with certain exceptions. It kinda makes some sense for books - there are many great recent books, but we're not going to stop reading Anne of Green Gables or Little House on the Prairie any time soon, I hope.
In other media, if you discount movies shared as rituals or keepsakes (nostalgia), you're left the timeless classics. One therefore hopes that the games that endure are timeless classics.
In some sense, Chess is a timeless classic. But it's really not more of a timeless classic than the same game on a 10x10 board with two "Prince" pieces on each side. Monopoly is a timeless classic, but there are tons of ways to make it better, and tons of games that build on the concept and make it better.
A large number of people stick with what they know in board games, but not with other things in life: video games, music, movies, clothes, books, and so on. This resistance to evolution is troubling and unnatural.