For the most part, you can blame the government. If OpenReach was still tax payer owned, the tax payer would pay to dig up the roads, install new ducts, upgrade to newer technology and so on.
Unfortunately it's not government owned, it's a company that has to make a profit, and ploughing hundreds of millions of pounds in to upgraded infrastructure for areas lowly populated doesn't make sense because they'd never see any ROI.
New builds are another story; I 'think' it boils down to House Builders not engaging early enough, though I'm not 100% on that.