1 - consumer freindlyness, adds complexity and cost. This appears to be BTs primary market for FTTC. With FTTPoD been aimed at the small business market.
2 - BT seem to not feel a need to market US speeds of higher than 20mbit/sec and a speculative guess has the bulk of infinity customers on the 40/10 not 80/20 service (this is backed up by that sky dont seem too bothered to sell their sky fibre pro product), so lack of demand for speeds higher than 40/10.
3 - Extending reach probably not their priority at the moment with the rollout still in progress.
I think vectoring needs to be seen to be effective first and foremost, as pair doubling adds crosstalk, there is stories eg. of people having 2 FTTC lines in one property and when the 2nd is activated the 1st loses a ton of sync speed.
With BT they are not frontrunners on tech and dont roll out things just because they better, they need to see it as a profit making machine.
BT Infinity 2 Since Dec 2012 - BQM