It's an incredibly fast moving target. Just look at the GTK mess, let alone keeping a wrapper toolkit up to date with other vendors' OS updates. Plus for operating systems like Windows it's not really clear which GUI technology to wrap. All the wrappers I've seen so far wrap the old school Win32 API but many modern applications are written in WPF or WinRT. Supporting all three would be difficult.

Also OS X is a somewhat different beast in terms of human interface guidelines and form layouts so it's really tough to come up with a unified abstraction layer that both supports new operating system features (and UI paradigms) and still provides a reasonably-sized API.