It's not surprising to see that AppleScript Studio has somewhat gone defunct. I mean, who actually uses AppleScript at all these days? Yet, on another side, it's somewhat sad to see it go away. No, Apple didn't kill it; its users did.
If you take a look at the AppleScript Studio programming guide, you can see that the last time it was updated was on 4 April, 2006. Soon, that'll be three years ago. There are plenty of reasons people may strand away from AppleScript onto other scripting languages such as Python, Perl, Ruby, and the like: it's not open-source, it's not cross-platform, it's not localized into other languages, and the list goes on and on. Moreover, scripting languages are rarely used to write full binary applications, but rather more utility-like ones that are usually interpreted.
Why is it sad that AppleScript, and more precisely, AppleScript Studio, is dying? It was the Rapid Application Development toolkit of the Macintosh. It eased the manipulations of other applications from your own app, and it allowed even non-experienced programmers to write simple applications. It could even be considered an intermediary step from little or no Macintosh programming experience into the world of Cocoa. After all, it's got all the same controls with similar methods and the same connection paradigm in Interface Builder.
Besides, it can't be forgotten how English-like it is; although that could be considered a weak point, I can imagine localization to other languages, which could, however, be hindered with grammatical differences, such as, for example, noun genders and contractions of prepositions with articles in Romance languages.
Overall, I can see many reasons why AppleScript Studio isn't the development toolkit of choice for programmers, but a lot of things that can be done with it would only be made more difficult by other means. I could see a bright future for AppleScript studio if Apple marketed it as heavily as they market Cocoa.
Note: by "Cocoa" in this article, I mean to refer to programming Cocoa via Objective-C, since AppleScript Studio applications are also Cocoa ones.
Update (13 Dec. 2008): After giving it a bit of further thought, I concluded that AppleScript and AppleScript Studio weren't exactly killed off, but Apple reduced marketing of it and thus reduced its overall presence.