Given that most people are consumers they are most likely to shop on price, not facility. All of those may possible FTTP clients also means lots of backend required at the exchange to serve more throughput, which just isn't there. so there's no point offering an upgrade to a service that can't be delivered....would require putting a lot more backhaul in that requires more investment in both terminating and authenticating equipment and switchgear but also backbone connection to main network trunk entry points.
Bottom line - they can make more money out of milking the existing provision and customers, with no further investment, while selling FTTP at a premium to a small number of customers (minimise investment, maximise return)....which will continue to be seen as an elevated premium service while it is not ubiquitous. Yet more evidence that the market will provide.