Blogger is now officially available in two different versions - Old Blogger & New Blogger.
While a majority of old blogspot blogs have migrated to the new Blogger, there are loads of bloggers still waiting for the Google invite that would allow them to shift from the old to the new version of Blogger.
Well, it turns out that Google has decided to postpone shifting of large blogs with thousands of posts to the new Blogger for some time.
Here's an official answer from the Blogger FAQ spotted by Improbulus and Impudent:
Why can't I switch to the new version of Blogger?
..if you have a very large blog (more than a couple thousand posts + comments), you'll need to hold off for a bit.
And that could also be the reason why DI is still on the old Blogger. The wait just got longer.
PS: Despite all the migration issues, it is still a one-way process. Once you have moved from old blogger to new blogger, you cannot reverse the path. There's absolutely no way to return to Classic Blogger from the New Blogger.
Update: Ionut Chitu shares a simple hack to switch from Old blogger to new blogger even without an invite. Here's how:
Sign out of Blogger and click this URL blogger.com/migrate-login.g - type in your old blogger credentials and then link them a Google Account.
If you don't sign-out completely before clickly the above URL, you'll see an error saying "Unfortunately, we cannot switch your Blogger account at this time, because one or more of your blogs cannot be moved."
Related: Google Eric Case on Blogger Future