'cause I'm not a coder.
And therefore I get silly ideas once in a while. 
EDIT: but still, Javascript can be run, right? So you could port the Installer to javascript... I just don't like the lag when I click on folders etc in the Installer; it shouldn't take a full second to switch to a parent directory etc. That and I get the feeling that it's taking the long way around when it gets lists and stuff. (Although that may be the best way available, like I said, I don't code so I don't know.)
That lag is entirely due to the way the the FSO installer was written, and nothing to do with language that it was written in. Goober5000 is in the process of rewriting the FSO Installer to improve it (I don't know the specifics). We may possibly have Desura as an installer in the future. wxLauncher will also become a launcher at some point.
All four possibilities all require coder time (Desura would probably be the least (as far as SCP coder time anyway)). Also in regards to porting to V8 I have no idea what kind of GUI and network library support that it has which would be critical writing an installer with it.
Nope, javascript and Java are too different to make convert applications between them. To elaborate, javascript is code that is run in your browser. The very last thing you want is javascript running on your system with the privileges needed for an installer-type program; there are very good reasons why javascript doesn't have access to any vital areas of a computer, most of which have to do with evil people infecting computers with viruses.
Well, the point of V8 is that you can use it to do things like you can with Java, and as such is no more insecure than running a Java program (they all have permissions of the user they are running as). That is, so it doesn't have to run in a browser. The primary reason that Google wrote V8 was so that they could write application servers like you do with Java or any other language, but write them in Javascript.