Because BT have better customer service, not brilliant mind you, but better than anyone else.

Call18866 doesn't allow you to contact them by phone and I know of at least one person who has had problems with Onetel's internet service. Besides that, Onetel's guarantee to be cheaper than BT won't apply to me - my call volume is sufficiently low as to qualify for the light user scheme (the only reason I don't qualify for it is that I have a mobile phone) and if your call volume is that low you don't qualify for the guarantee.