The initial beta design had so many obvious issues that it's wild that it made it as far as it did. Hell, the readability of many UI elements was obviously terrible in the initial reveal, where you'd expect everything to be shown in the best possible light.

Obviously Apple can improve things for the final release (and it seems like they're taking some steps in that direction). But these issues should have been identified long before the beta was released, and the fact that they weren't does not inspire confidence.

The first beta often ships with core features missing or broken. It exists to get as many new features in front of third party developers as soon as possible, because Apple has very little time to accept feedback before they are locked in for shipping.

At the same time, there seems to be precious little time between when Apple decides a feature is going to ship in the next release, and when WWDC happens.

Even if there was common knowledge inside the company that a new UI was coming, it may have not been merged into mainline until closer to WWDC. At that point, individual teams will need to alter their code to build and be usable on top of the UI as part of continuing their own development - but were likely still focused on the death march for their own WWDC-launched features.