This can be due to one or more different factors.
Your line may be connected to a roadside cabinet that has not yet been upgraded to fibre broadband. As the rollout progresses, more and more cabinets will be upgraded.
You may be served by what is known as an 'exchange only' line that is connected directly to the telephone exchange without an intervening green roadside cabinet. Please see the relevant question about exchange only lines.
You may be on a line that is too long to support a fibre broadband connection from your local roadside cabinet.
We’re working hard to upgrade as many premises as possible to fibre broadband, and where fibre optic cannot be made available we’re working on finding the next best solution.