Because it stays open in its own browser tab and I don't have to open up a separate application to post.
I like that. It makes for a happier bunch of CPU cycles, and a happier Jenn.
Scribefire still needs work. It would be nice if it had LJ tags in a dropdown, and if it let you select moods and icons, but that's probably asking too much.
If anybody knows of a Firefox extension designed to work with LJ posting which has those features, do tell.