The million dollar question….
We've been banging on the proverbial doors trying to find out why the known EF 200-400 f/4L IS 1.4x has yet to be officially announced, priced (it's going to be $10,000+) and made available to the people that need it.
The latest we've heard, and from a pretty knowledgeable person is that the lens has needed a redesign in regards to the “bump” that holds the built-in 1.4 TC. The person wouldn't elaborate as to why it was an issue, but it was brought to the attention of Canon at the Olympics that some pros were complaining that the “bump”, or a function of the “bump” got in the way of something. Again, what the something was wasn't elaborated upon. If anyone out there knows, I'd love to hear it.
This is about the only explanation I have received beyond “manufacturing issues”, which could also be playing a small roll I suppose. However, lately we've seen that the new super-telephoto lenses are now making their way to “in stock” status around the planet. Maybe we're closer to the new supertelephoto finally being announced.
Sorry we can't be more conclusive on this one, this is the best information we have.