Because .UK only allowed registrations at the third level for decades, i.e. example.co.uk, example.ac.uk etc. I think it's also the most-used CCTLD. Then they

Because .UK only allowed registrations at the third level for decades, i.e. example.co.uk, example.ac.uk etc. I think it's also the most-used CCTLD. Then they introduced second level domains, but they're still rarely used at present. ebay.uk and google.uk are registered, but don't do anything yet.