Why are many sequels so disappointing?
Lack of development money as compared with the original.
Company that made the original bought out by a different company that has a mistaken idea of what made the original game good -- or that simply doesn't care and only wants to throw something together as cheaply as possible to extract money from fans of the original game.
Company that made the original gets new management which thinks they can make more money by changing a successful formula -- and proceeds to make a huge botch.
Original game a "one hit wonder" from developers that were never able to repeat their success because they felt they had to add unwanted "features" to the sequels in order to be "relevant to today's gaming" (whatever that means)
Obsession with 3D, despite problems. Developers only seeing the potential, unproven adventages of 3D engines and being blind to potential issues. Bugs being much worse than expected. Development time allotted for debugging woefully inadequate. Release of games sorely in need of patching. Release of games with extremely high system requirements for how the game looks and plays -- so you get a chuggy, buggy game that looks worse than its 2D (or 2.5D) predecessor, even on computers that play modern RPG's without difficulty.
Obsession with supporting all possible game platforms, which means using certain game engines and expecting the gamer to endure their quirks, like lack of true point-and-click controls and ugly prefab interface.
Obsession with gimmicks, like Achievements, Facebook/Twitter/Steam integration, and gameplay borrowed from casual games -- things many of us consider negatives -- especially when they replace the more traditional adventure gameplay that was what drew us to the adventure genre in the first place.
Take a look at Still Life. The first game was rather successful and many of us clamored for Still Life 2, which we eventually got. However, the producers of the game had changed it.
The developers of the original Still Life game no longer existed as a company at the time Still Life 2 was made.
The development arm of Microids, Microids Canada, which developed the original Still Life, was one of the victims of publishers non-payment of developers for their games. Microids Canada was sold off and the individuals who worked on Still Life ended up working on action games for Ubisoft, or finding work elsewhere. The team that worked on Still Life was never brought together again and never made another adventure game together. Still Life was the last game made by the real Microids.
Still Life 2 was developed by a completely different company, GameCO Studios. The publishing arm of Microids continued to exist for a few years longer before it also went under and was purchased by Anuman around 2009 or 2010. Not that it mattered. All they were at that point was a name.
Buying a bankrupt company and publishing games under that company's name does not automatically make the games resemble the original company's games. Not in the slightest. The development arm of Microids has been dead since 2005 -- and that's the one that actually made
the games as opposed to publishing some other company's games.