My personal feeling is that the SDK isn't the problem, Apple is. I suspect that TomTom aren't too happy about having to pay 30% of the profits to Apple. Considering that TomTom Navigator 6 (which is about 3 years old now) is £90 (and was about £120 at release) they could either hike the prices up to cover the loss which would put a lot of people off as its already quite expensive, or potentially sell it at a loss (considering the price of the the maps licences).
Another problem I can see is that the iPhone only allows 1GB of space on which you can install apps (you can increase this by jailbreaking). Now consider the size of the maps. I think the UK map was over 300MB, and we're quite small compared to some other countries. Then the app itself was about 100MB on top of that. I can see space on the device being a problem too.