Cabinets enabled under the BDUK projects (and some of the new cabinets in Wales may still be part of the commercial roll-out remember) are done on a gap funding basis and the budgets are fixed, so it makes sense for the roll-out to be doing the ones which are closer to being economically viable.
With the clawback mechanism if these early cabinets reach commercial take-up levels, BT may have to give some money back. Which could then be recycled to do those harder to reach cabinets that they thought were not viable originally.
it is a very moveable feast, in part because you don't know all the costs for an individual cabinet, e.g. power until you do all the paperwork and power company reply. Then you get cases where people object to location, and it has to be moved, or fingers crossed this is caught in planning stage when its just a day or so of paperwork wasted.