I think unpopularity is a little bit harsh, they are not anymore the focus point in the industry but Adventure Games still sell good at germany (not every one, even good ones get slapped and its a hard world but still, you can develop games and make profit). You will most likley know but Adventure Games where pioneers for graphic technology in their golden age.
For today, other genres took over, in the 90', the FPS began to take this task and other genres began to develop very nicely while adventures stopped more or less in their development. You could argue adventures reached their design zenith but it would miss the point and i will tell you why i think so.
So the biggest problem i see for today adventures is, they simply stopped to develop at some time and the fundamental gameplay did not change very much or new gameplay developed during the golden era where simply overlooked AND the huge amount of bad games flooded the market (exspecially the live action video era with 16 CD-ROMs) and killed adventures as a dominant genre more or less. While adventures lost slowly their momentum, at some point the RPG genre began to addopt gameplay parts from adventures. Dialog trees, simple item puzzles, etc. Modern Bioware games are more or less the modern reincarnation of adventure game in my eyes. Take down all the battle aspects of Biowares games since KotOR (while Planescape Torment started this development) and you get an very rich modern adventure. Bioware even said the time for action slowly fades away, so i think maybe Bioware could give the adventure genre new impulses. Instead of one predetermined path, you get several possible ways of solving puzzles or quests, while your action will change the outcome of the story and behaviour of NPCs.
Personally, i see the future of adventures not in Telltale games, Still Life or Secret Files, and while they are fun, i see the real future in games like Knights of the old Republic, Mass Effect and The Witcher. Sleeping, developing, waiting for someone to free them and rebirth in a new way.
This is my analysis why they lost popularity, i could be totally wrong but at least im very sure we stand at a turning point for adventures:
http://www.gamesindustry.biz/articles/bioware-mature-industry-can-drop-violence-from-games