Two reasons. First, the only "critical" bug I'm aware of with 0.6 (aside from redirecting to the wrong page sometimes) is the lack of pagination, which requires a number of fixes for URL routing to get working.
Second, I wanted to remove the "mod_rewrite" requirement and start introducing some basic concepts like categories and plugin hooks.
Even if I just concentrated on the first issue(s), I would have needed to make some significant changes to the code.
In the end, I elected to simply work everything into a complete refactoring of the codebase instead. In hindsight, this may not have been the best decision. At this point though, v0.7 is so close to having a stable release that I don't want to split my time between two code bases.
All I can do is apologize for making everyone wait for this release, but I believe it will be worth it. The functional changes made to the core system make the whole application faster, more extensible, more reliable and more customizable.
I believe I can publish a release candidate by Tuesday (July 6), get feedback for a week while I'm in L.A. and San Diego, then a final release about a week later. This will be the version that gets published to GitHub and will form the basis for the upcoming v1.0 release. There will be a complete upgrade path, so if you're currently using the 0.6 or 0.5 versions, you won't lose anything :)
Thanks for the update Rust