Does it even matter? I can't think of many valid reasons for delaying a feature for 11+ months that was supposedly very close to making the release version of GT6. Not without some sort of communication at least.
The valid reasons I can think of mostly fall under the categories of "random natural disasters" and "spectacular and deadly acts of God". Either of which tend to turn up in the news, so we probably would have heard about it.
I think what's actually delaying it is one of two things:
1. They announced the feature before they actually knew whether it was feasible. Turns out that it's kind of not, or at least it's orders of magnitude more difficult than they expected. Thus having painted themselves into a corner, they continue to try and actually make the feature work not knowing whether they'll ever actually get it functional. (Yes, I'm aware that Kaz says he has a beta version. He also said that it was nearly ready in December 2013. The man talks in riddles and fibs. What he calls a beta version could be barely functional from a commercial perspective for all we know.)
2. Gran Turismo 7. They've put a skeleton crew on supporting GT6 and everyone else is working on GT7. The lack of progress makes a lot more sense if there's only one guy actually working on the feature.
Neither of which are acceptable reasons to give to paying customers.
The course creator was made by one person in GT5; how do you "skeleton crew" that? There's no credit for GT6, funnily enough.
Programming is not content. It was also very likely well under way before they announced it.
You have to remember that they are targeting both the PS3 and PS4 with all these features (for eventual efficiency, at the expense of up-front effort), so you're probably right, but not in the way you explained. Please consider the issue of incremental updates for something that is inherently interdependent with other systems (and the "patch breaking a previous patch" phenomenon prolific in software). Once again, the course creator is a suite of technologies, not just the interface we use to "make" the courses themselves. All software is more complicated than initially anticipated; try it, it's a wonderful insight for a scientist to gain.
Taking the decision to consolidate development and delay it further is not a misjudgment of "feasibility", it's damage limitation. Besides, features get axed during game development all the time, only game development is becoming more and more "patch after release". That's good when intentions are good (you eventually get a game closer to what the developers wanted to make, assuming you can update), and bad otherwise (release a game "early", incomplete, cynically). Of course, no game is black & white.
There's also the community features to consider; namely in the sense of security, or the fact that the PS3 has no security any more. That's why everything the game holds in memory is encrypted and only decrypted exactly at the moment it is needed, and only in part, or that PD have added extra encryption to their saves, or that PD changed the encryption they were using for the file structure back in the spring. Not only does that complicate the interaction of the game's systems, but there is surely no doubt the course creator will have a "community" aspect, as would the sound update (eventually) and B-Spec.
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