A sad fact of gaming is that the average release has a lifespan that'd make even a mayfly sympathetic. The average game hits the shelves on Monday, the bargain bin on Wednesday, and by Friday, it likely lives on as nothing more than a fond memory and a few references on the geekier online forums.
Publishers rarely do more with their back catalogues than try to prevent anyone distributing them, stores don't have the space to stock games that aren't selling great guns, and gamers, well, gamers are fickle.
Just a couple of years can be the difference between the Best Game Ever and outright torture on a neatly pressed CD. The absence of basic features like high-resolution, mouse-look, or the ability to jump can turn even an acknowledged classic like Doom into nothing more than a rose-tinted amble for old-school players, while newer players try to work out just what all the fuss was about.
And then there are the remake projects. There are two basic types of remake; what you might call reconstruction and re-mastering. A reconstruction takes a basic game concept, such as Frogger or Space Invaders, and tries to modernize it. A re-mastered game focuses on keeping the core gameplay more or less as it was, but bringing the technology (and typically interface design) to modern levels. An example of the former might be a 3D version of Frogger, where the camera is in the frog's mouth instead of hanging over the level, or where the frog has to navigate a multi-level 3D world to get to its destination. A re-mastered version would simply swap out the old sprites for polished 3D models and leave the rest alone.
Commercial Remakes
Very few companies have bothered with either, for a number of reasons. Most notably, there's not much money in it, unless you're porting a game to a completely new platform and a new audience. Even if the game has been radically overhauled, the perception that it's old can work against you, and the number of people desperately hankering for a second crack at one particular title is rarely going to match up to the potential audience of something shiny and brand new.
Services like Xbox Live have shaken this up a bit, as happened when Prince of Persia got both a shiny new game in the form of the Sands of time trilogy, and a full remake of the original plat-former, but this rarely happens. That said, there have been some high profile commercial remakes. Most recently, the heavy hitter was Tomb Raider: Anniversary, which took the original game's plot and basic locations and rebuilt them. As well as having more polygons and graphical effects to splash on Lara and her environment, each area was designed with the idea of creating the game Core would have made, had they had modern technology and ten years of sequels behind them back at launch. This included far more intricate puzzles and scenery, a Lara capable of moving freely around the world (the original used fixed block units, and access to triangles didn't occur until Tomb Raider III) and QuickTime events during cut-scenes and big boss fights.
Most remakes aren't lucky enough to have such major licenses behind them, and the games that companies choose to grace with a new version can be... unusual. RealMyst - a 3D recreation of the original Myst game - is fair enough.
The Journeyman Project? Not so much. Westwood called in outside assistance for Dune 2000 - easily it's most forgettable RTS. Sid Meier's Pirates travelled from 1987 to 2004 and picked up nothing but new fans. And of course, some games should just have remained buried. Any good feelings people had towards Defender of the Crown went right down the privy after actually playing it in the cold light of good gaming. Goodness only knows who was clamoring for an update of Bad Mojo, the cockroach simulator/adventure in gross-out graphics, where dead rat and chopped up catfish backgrounds were lovingly reintroduced to our stomachs.
Arguably the most impressive commercial remake of all time was Tex Murphy: Overseer, released in 1998. This was based on the 1989 adventure, Mean Streets, although about all they had in common was the basic plot framework. Mean Streets was an odd mix of side-on adventure, flight simulator bits, and very primitive attempts to get actors (or to be more accurate, employees of Access Software) into a game.
Overseer's story was completely re-written, setting it in a full 3D world that could be explored at will, with all the characters and cut-scenes handled by some surprisingly good full motion videos. It was one of the last interactive movies. It, didn't so much bomb as nuke itself - the UK release going straight to budget.