Because licensing them means they miss out on their $39.95 per cable.
An alternative explanation (as they could raise their licensing fees instead, and the costs of managing licensing fees is much lower than constructing and distributing cables; when you look at their profit report these cables are just not a major part of their business) is that they found that third party cables often had minor aberrations that damaged the contacts on the device, leading to increased warranty repair…
This is their first new dock connector since 2005. They may want to see it (or third-party accessories for it) in the field before approving third-party adapters. There may be electrical details that are tricky to get right. And so on.
Normally I'm quite willing to believe the worst of Apple, but here there are too many reasonable explanations for limiting third-party adapters and cables for the moment. If the limitations end up not being temporary, that would be another story, but until then...