One of the issues always was that Google would give no guidance on how upgrading from .co.uk to .uk would effect SEO, or a gold standard way of doing it. Nominet did try to get some feedback from Google, but they wouldn't give any, they refused to answer any questions.
Many companies love the idea of .uk but put off by the risks of moving a successfully ranking site. At the moment, it's mostly newly launched sites that are using .uk. It will change at some point and .co.uk owners will be forced to make the switch, when that point is though, who knows.