As far as I know, it's the same issue with all DSLR:s. Normal TTL (with film cameras) uses light bounced off the film, usually with a sensor in the bottom of the film chamber. The sensor in a digital camera reflects light differently. It acts more like a mirror, reflecting light back at the lens. This means you can't have an efficient sensor without being in the way of the light. Therefore, DSLR:s employ different variants of a pre-flash. It's measured through the lens (so it is some kind of TTL), but it requires different capabilites of the flashes. For one thing, the flash has to be able to deliver two flashes in a close period of time (about 1/10th of a second). Second, and more importantly, the flash has to control its output differently. Instead of "just flash and I'll tell you when it's enough", the flash has to do first a pre-flash at some known power, and then when the camera says so, "do a flash that's 1.4x the power of the last one". It requires a completely different communication protocol, and more precise controlling electronics (being able to fire flashes with consistent, known power).
However, the more I use my camera, the less I feel the need for an external flash. High sensitivity, AS, a couple of f/1.8 primes, and the internal flash, together mean that I've never really felt that a more powerful flash has been necessary so far. Also, in most indoor situations I really don't want to use a flash anyway.