Because it is provided over existing copper wire infrastructure, traditional broadband is highly available across the whole of the UK (pretty much) and allows for decent and usable interaction with every type of service (up to a reasonable level of quality) that the internet has to offer. Broadband can deliver video, voice and all types of data to offices and homes and for many is still the preferred connection of choice.
The service is based on a contention ratio (how many other connections share your bandwidth at any given time) of 20-50 times. This means that you are competing with 20-50 hosts for the bandwidth reservation your ISP has reserved on your behalf. The upshot of this is that the ISP only need charge you 1/15th pf the cost of that bandwidth and still return a tidy profit and so the cost of this type of connection is extremely cheap.
The connection being based on asynchronous transfer management (ATM) also allows for the ISP to cache a lot of the traffic demand locally rather than sending out every request to the internet proper. If 100,000 users are trying to access BBC iPlayer that evening then the sensible thing to do is to download a copy of the site every hour and supply that copy to users across the ISPs internal network and reserve the actual internet traffic for those uploading or requesting more unusual searches etc. This enables the ISP to optimise their system for broadband users and provide a highly usable and reliable service.
As there is no surveying or planning to be made when installing the cabling - it is there already - then the amount of lead-time for installation is substantially less. A dedicated fibre connection is usually scheduled for 90 days in the UK but EFM can be rolled out within 21 days as the system only has to be configured or installed at the exchange or ISP POP, not every inch of the way between you and them.
Due to the lightweight nature of the arrangements that need to be made in order for EFM to be installed there is a far smaller risk to be incurred on the part of the provider. As a result the term of contract is usually much shorter and more open to renegotiation or change.