HbbTV is based on MHP, which is a Java-based system. Freeview uses the MHEG-5 object-description language with the UK Profile of objects that the receiver must implement. Switching Freeview to use HbbTV would have to completely duplicate everything already carried in MHEG, which would reduce the amount of space for TV channels. A couple of years ago, the Digital TV Group (which writes the specification for UK receivers) added an extension to the UK Profile called MHEG-IC (Interaction Channel), which defines how the receiver can locate IPTV services.
Saorview receivers must implement MHEG as well as legacy teletext, as well as everything in NorDig specs. Freeview receivers are not required to implement legacy teletext, which is not carried in UK broadcasts.
All Freeview-branded equipment is required to implement MHEG and UK Profile, and I believe any box with an Ethernet port is required to implement MHEG-IC. Freeview HD boxes already implement the features, with several virtual channels (i.e. where the broadcast only contains metadata pointing to the IPTV service) having already launched. Compatibility is a problem though - see http://www.visiontvnetwork.co.uk/home/devices for a list of compatible devices.
YouView is an attempt to fix the compatibility problem. Freeview manufacturers can and do develop their own software stacks, but that means that anyone wanting to provide a catch-up service, rather than just live streams, has to develop a different app for each different TV or box. Even trying to use a web app often fails, because each TV's firmware contains a different version of the web browser. YouView is trying to standardise what the stack is, somewhat like Android, to ensure that an app will run on any YouView box. YouView will be providing substantial parts of that stack. I believe the delays have been in developing that stack and getting receiver manufacturers to integrate it - the final core specification for launch was finished over a year ago.