Not exactly true, the non-BT option is primarily Virgin. Virgin has a pretty good infrastructure (at home I have 50Mbps broadband from them) and generally they are cheaper. BT on the other hand is more expensive. If available I'm sure businesses would select Virgin. But we (I work for one business that is stuck with BT) have no option but BT because Virgin doesn't have its fibre in our region.
While BT runs over telephone wires. Therefore is more pervasive in the UK.
I work for one of those businesses and we were affected. The reason we didn't have a backup line, is because the other major ISP (Virgin) doesn't have its fibre in our region. Therefore, BT is our only option.