Graphic adventures just don't sell anymore, and thus the cost of developing one isn't justified. The text adventure went the same way, oh so long ago (man, late '80s wasn't it? I played quite a few of them on my C64 and Amiga), and it stood to reason that the graphic adventure would follow at some point.
Games like Runaway, Syberia, The Longest Journey and the upcoming Leisure Suit Larry game show that people will still buy a well done and fun adventure game.
Also, I'm pretty sure Monkey Island will get another sequel. The difference between Guybrush and Sam and Max (or Full Throttle for that matter) is that they are cult classics. Monkey Island easily went above that, and is a decent sale for LucasArts.