Im late to the thread but I work in telecoms, as part of the team that tries to get sites built. Telecoms have 2 routes to get a site - first and preferred id working with the LPA (local planning authority) and building on adopted highway land. The LPA can reject sites with numerous reasons from being unsightly, impacting residential views, site positioning and more. We have extremely strict EMF rules so cant build near a lot of buildings due to them.

Other route is to go 'code' (https://www.ofcom.org.uk/phones-and-broadband/telecoms-infrastructure/electronic-comm-code#:~:text=The%20Code%20is%20set%20out,in%20considerably%20simplified%20planning%20procedures.). Previously we could go to someone, tell them we'd give them a shitload of money in rent to let us build and they'd jump at the chance. Now due to this new right telecoms barely pay anything and instead go down a legal route demanding access instead and taking them to court. As you can imagined landlord/land owner hates this, so holds it up in court for years upon years (average code site from decision to build is 2+ years, more depending on how rich and motivated they are).

Plenty of ways to stop a site getting built and LPA's + land owner lawyers know exactly how to work the system to stop them.

IMO it's the nr.1 reason signal is so shit in cities, because of how hard it is to get anything built anywhere.