Puzzles were difficult, natural language processing is a hard problems, and commands/grammers could get weird/complex. The icon based point-n-click cleared this up a little, but Sierra had a business model build around needing to order hint books (or play with large groups of people).
No one wants hint books, because once you use one hint, you tend to just keep using them. With the Internet and things like UHS for adventure games, this model kinda hit a wall.
Telltale, Double Fine, et. al. brought this gene back by treating it more as an interactive story. The puzzles were less crazy and you could get through them without a hint guide.
Broken Age is a stealer example of both versions of this genre. Part I is more like Telltale, except with better puzzles. It's easy to get through, but still challenging. Part 2 goes back to the Sierra model though, with puzzles that make little sense and are insolvable for most casual and moderate gamers without a hint guide.
