Eel's discount consultancy says..
Cost, and policy.
So it would cost a lot of billions to roll out fibre everywhere. It would also be extremely disruptive, and policies get in the way.
So in a perfect world, there'd be clean ducts to every property so you could blow or pull fibre. Then at an agreed date & time, cut 25m+ households over from copper to fibre. That assumes customers are in, have done prep work like making sure space and power is available.
Reality is ducts are full of copper, roots, collapses and don't have enough space to build in parallel. So either big service disruption while that's fixed, copper's pulled and fibre's installed, or massive civils costs and road disruption to dig in new ducts. Plus of course sorting out wayleaves.
So then you've spent thousands connecting a new customer. How will competitive access, ie LLU-style services work? Will the customer, or new operator have to pay the original install costs + modest ROI before services are ported? If there's some form of investor protection, that'll just create local monopolies and network islands, which might not be good for consumers. A USO model where all operators kick in to an infrastructure fund to pay for rural services might work, but experience from places like the US show that's gamed, or competitors don't want to pay. Or the costs just get paassed through to consumers, increasing their bills.
That's a subsidy issue, which is also governed by national and international law. There's some wriggle room around services being part of critical infrastructure (safety of life being one, ie making an emergency call). But needs a political decision and firm control over the market (ie incumbent(s) + competitors) to make it so, and prevent the usual regulatory shenanigands to give someone a competitive advantage.
Then there are other political challenges. So councils (not just in the UK) don't like roads being continually dug up to install ducts and access chambers. So councils can put 'stops' on roads that prevent new installations for a few years.. Which is a problem in big cities like London, Paris, Frankfurt etc. We'd love to give you new fibre, but can't touch the roads till 2023.
Again that's a political decision, so costing up a full UK fibre rollout, figuring out competitive access, acceptable ROI and OAM costs per connection and then planning the 'Big Dig' to make it happen.. Which is a lot easier said than done.